feat(model): FlowGraph + LayoutStore foundation for WYSIWYG editor

First piece of the v0.2.0 rewrite. The graph view, text view,
and run view will all share a single source of truth: the
YAML text. This commit introduces the parsing layer + the
sidecar that keeps layout metadata OUT of the YAML.

FlowGraph (lib/src/model/flow_graph.dart):
 - Parses + emits the F-Delta-I flow YAML shape (inputs +
   steps + outputs + leading comment block).
 - Recognises both inputs shorthand (`name: text`) and the
   expanded form (`name: { type: text, default: ..., hint: ... }`).
 - Detects edges by scanning step.with + outputs for
   $source.field references; understands both the short
   $x.y form (canonical) and the long ${{ x.y }} form (legacy).
 - Flags system.approval@ steps via FlowStep.isApproval so
   the graph can paint them with the human-gate styling.
 - Immutable update helpers (withStepUpdated, withStepAdded,
   withStepRemoved) used by the property panel + add-step
   button.
 - tryParse returns null on YAML syntax errors so the text
   tab can keep the last valid graph while the operator
   types.

LayoutStore + FlowLayout (lib/src/model/layout_store.dart):
 - Per-flow JSON sidecar at
   ~/.fai/data/flows/.layout/<name>.json. Atomic write via
   tmp + rename so a crash mid-save leaves the previous
   copy intact.
 - The YAML stays byte-identical to what `fai run` consumes
   — positions never bleed into the flow file.

AutoLayout (lib/src/model/auto_layout.dart):
 - Deterministic topological column layout for flows that
   have never been opened in the graph view before.
 - Steps with no $step refs sit in column 0; steps whose
   refs all point at column-0 steps sit in column 1; etc.
 - Cycles + dangling refs collapse to column 0 so nothing
   is ever invisible.

Tests (test/flow_graph_test.dart): 7 cases cover canonical
parsing, edge detection across both ref forms, approval-step
flagging, leading-comment round-trip, structural round-trip,
parser robustness on broken YAML, immutability invariant.
All pass.

Version 0.1.4 -> 0.2.0 (minor bump — significant new module
graph; public FlowEditorPage constructor API unchanged).

Signed-off-by: flemming-it <sf@flemming.it>

lib/src/model/auto_layout.dart

@@ -0,0 +1,100 @@
+// Auto-layout — deterministic node placement when no sidecar
+// positions exist yet. Pure function over the graph; called
+// every time the graph view loads a flow that has no saved
+// layout.
+//
+// Strategy: light-weight topological columns. Steps that
+// can run first (no $step refs in their inputs) sit in
+// column 0; steps whose only refs point at column-0 steps
+// land in column 1; and so on. Rows within a column wrap
+// vertically so wide graphs scroll horizontally and tall
+// columns scroll vertically.
+//
+// This is intentionally simple — operators are expected to
+// fine-tune positions by dragging once a flow is open. The
+// auto-layout exists to give a non-overlapping starting
+// point, not the final visual.
+
+import 'flow_graph.dart';
+import 'layout_store.dart';
+
+class AutoLayout {
+  // Box geometry in canvas pixels.
+  static const double nodeWidth = 220;
+  static const double nodeHeight = 110;
+  static const double hGap = 80;
+  static const double vGap = 40;
+  // Origin offset so the leftmost column doesn't hug the
+  // canvas edge — leaves breathing room for the special
+  // "inputs" sidebar.
+  static const double originX = 320;
+  static const double originY = 80;
+
+  /// Produce positions for every step in [graph], filling in
+  /// any not already covered by [existing].
+  static FlowLayout layout(FlowGraph graph, FlowLayout existing) {
+    final cols = _assignColumns(graph);
+    final byColumn = <int, List<FlowStep>>{};
+    for (final step in graph.steps) {
+      final col = cols[step.id] ?? 0;
+      byColumn.putIfAbsent(col, () => []).add(step);
+    }
+    final result = <String, NodePosition>{...existing.positions};
+    byColumn.forEach((col, stepsInCol) {
+      for (var i = 0; i < stepsInCol.length; i++) {
+        final step = stepsInCol[i];
+        if (result.containsKey(step.id)) continue;
+        final x = originX + col * (nodeWidth + hGap);
+        final y = originY + i * (nodeHeight + vGap);
+        result[step.id] = NodePosition(x, y);
+      }
+    });
+    return FlowLayout(positions: result);
+  }
+
+  /// For each step, the 0-based column index it belongs to
+  /// (longest predecessor chain + 1). Steps with no
+  /// dependencies sit at column 0.
+  static Map<String, int> _assignColumns(FlowGraph graph) {
+    final cols = <String, int>{};
+    final stepIds = graph.steps.map((s) => s.id).toSet();
+    bool progressed;
+    do {
+      progressed = false;
+      for (final step in graph.steps) {
+        if (cols.containsKey(step.id)) continue;
+        final preds = _predecessors(step, stepIds);
+        if (preds.every(cols.containsKey)) {
+          final col = preds.isEmpty
+              ? 0
+              : preds.map((p) => cols[p]!).reduce((a, b) => a > b ? a : b) + 1;
+          cols[step.id] = col;
+          progressed = true;
+        }
+      }
+    } while (progressed);
+    // Cycle / dangling-ref fallback — anything still
+    // unassigned gets placed in column 0 so it's at least
+    // visible.
+    for (final step in graph.steps) {
+      cols.putIfAbsent(step.id, () => 0);
+    }
+    return cols;
+  }
+
+  static Set<String> _predecessors(FlowStep step, Set<String> stepIds) {
+    final refs = <String>{};
+    for (final v in step.with_.values) {
+      if (v is! String) continue;
+      for (final match in _refPattern.allMatches(v)) {
+        final src = match.group(1)!;
+        if (stepIds.contains(src)) refs.add(src);
+      }
+    }
+    return refs;
+  }
+
+  static final _refPattern = RegExp(
+    r'\$([A-Za-z_][A-Za-z0-9_-]*)\.([A-Za-z_][A-Za-z0-9_-]*)',
+  );
+}

lib/src/model/flow_graph.dart

@@ -0,0 +1,436 @@
+// FlowGraph — in-memory model of a flow YAML.
+//
+// The YAML text is the canonical source of truth. The graph
+// view derives a `FlowGraph` from the parsed YAML, and any
+// edit in the graph view (drag a node, rename a step, add a
+// connection) is re-emitted as YAML and pushed back into the
+// shared text buffer. The graph never holds state the YAML
+// can't represent — everything visible can be expressed in
+// the YAML round-trip.
+//
+// Layout coordinates are the one piece that doesn't live in
+// the YAML; those are stored in a sidecar file via
+// [LayoutStore]. That keeps the YAML byte-identical to what
+// `fai run` consumes.
+
+import 'package:yaml/yaml.dart';
+
+/// One flow, fully decoded from its YAML text.
+class FlowGraph {
+  /// Flow display name (`name:` at the top of the YAML).
+  /// Optional — many example flows omit it and the file name
+  /// serves as the identifier.
+  final String? name;
+
+  /// Declared inputs the operator supplies at run time.
+  /// Each entry maps the input name to its declared shape.
+  final Map<String, FlowInput> inputs;
+
+  /// Ordered list of steps. Order matters for the linear
+  /// engine; the graph view preserves it on save.
+  final List<FlowStep> steps;
+
+  /// Outputs the flow exposes — each value is an expression
+  /// that references a step output or an input
+  /// (`$step.field` / `$inputs.field`).
+  final Map<String, String> outputs;
+
+  /// Free-form comment block at the top of the file. Captured
+  /// so the round-trip preserves human-authored context like
+  /// example invocations or compliance notes. We can only
+  /// preserve LEADING comments; mid-file comments are lost on
+  /// graph -> YAML emission (acceptable trade-off — they're
+  /// rare and the operator gets a warning if any are present).
+  final String leadingComment;
+
+  const FlowGraph({
+    this.name,
+    required this.inputs,
+    required this.steps,
+    required this.outputs,
+    this.leadingComment = '',
+  });
+
+  /// Empty placeholder while a YAML buffer is still being
+  /// loaded or has just been cleared.
+  factory FlowGraph.empty() =>
+      const FlowGraph(inputs: {}, steps: [], outputs: {});
+
+  /// Parse [yamlText] into a [FlowGraph]. Returns `null` if
+  /// the YAML can't be parsed at all (syntax error). For
+  /// semantically invalid flows that still parse (missing
+  /// step.id, etc.) the result is best-effort and any
+  /// recovered structure is returned.
+  static FlowGraph? tryParse(String yamlText) {
+    try {
+      return FlowGraph.fromYaml(yamlText);
+    } catch (_) {
+      return null;
+    }
+  }
+
+  factory FlowGraph.fromYaml(String yamlText) {
+    final leading = _captureLeadingComments(yamlText);
+    final doc = loadYaml(yamlText);
+    if (doc is! Map) {
+      return FlowGraph(
+        inputs: const {},
+        steps: const [],
+        outputs: const {},
+        leadingComment: leading,
+      );
+    }
+    final name = doc['name']?.toString();
+    final inputs = _parseInputs(doc['inputs']);
+    final steps = _parseSteps(doc['steps']);
+    final outputs = _parseOutputs(doc['outputs']);
+    return FlowGraph(
+      name: name,
+      inputs: inputs,
+      steps: steps,
+      outputs: outputs,
+      leadingComment: leading,
+    );
+  }
+
+  /// Emit a YAML representation. The output is intentionally
+  /// stable and human-friendly: 2-space indent, double-quoted
+  /// strings when needed, no flow-style maps. Designed to
+  /// match the hand-authored examples shipped under
+  /// `flows/` so the text view doesn't shock the operator
+  /// after a graph edit.
+  String toYaml() {
+    final b = StringBuffer();
+    if (leadingComment.isNotEmpty) {
+      b.write(leadingComment);
+      if (!leadingComment.endsWith('\n')) b.writeln();
+      b.writeln();
+    }
+    if (name != null && name!.isNotEmpty) {
+      b.writeln('name: ${_scalar(name!)}');
+      b.writeln();
+    }
+    if (inputs.isNotEmpty) {
+      b.writeln('inputs:');
+      for (final entry in inputs.entries) {
+        final v = entry.value;
+        if (v.hasOnlyType) {
+          b.writeln('  ${entry.key}: ${v.type}');
+        } else {
+          b.writeln('  ${entry.key}:');
+          b.writeln('    type: ${v.type}');
+          if (v.defaultValue != null) {
+            b.writeln('    default: ${_scalar(v.defaultValue!)}');
+          }
+          if (v.hint != null) b.writeln('    hint: ${_scalar(v.hint!)}');
+        }
+      }
+      b.writeln();
+    }
+    if (steps.isNotEmpty) {
+      b.writeln('steps:');
+      for (final s in steps) {
+        b.writeln('  - id: ${s.id}');
+        b.writeln('    use: ${s.use}');
+        if (s.with_.isNotEmpty) {
+          b.writeln('    with:');
+          for (final entry in s.with_.entries) {
+            b.writeln('      ${entry.key}: ${_scalar(entry.value.toString())}');
+          }
+        }
+      }
+      b.writeln();
+    }
+    if (outputs.isNotEmpty) {
+      b.writeln('outputs:');
+      for (final entry in outputs.entries) {
+        b.writeln('  ${entry.key}: ${_scalar(entry.value)}');
+      }
+    }
+    return b.toString();
+  }
+
+  /// Derive the edges implied by `$step.field` /
+  /// `$inputs.field` references found in step `with:` values
+  /// and in outputs. The graph view paints one edge per
+  /// reference.
+  List<FlowEdge> get edges {
+    final result = <FlowEdge>[];
+    final stepIds = steps.map((s) => s.id).toSet();
+    for (final step in steps) {
+      for (final entry in step.with_.entries) {
+        final value = entry.value;
+        if (value is! String) continue;
+        for (final ref in _scanRefs(value)) {
+          if (ref.source == 'inputs' && inputs.containsKey(ref.field)) {
+            result.add(
+              FlowEdge(
+                fromKind: EdgeEndpointKind.inputs,
+                fromId: 'inputs',
+                fromField: ref.field,
+                toKind: EdgeEndpointKind.step,
+                toId: step.id,
+                toField: entry.key,
+              ),
+            );
+          } else if (stepIds.contains(ref.source)) {
+            result.add(
+              FlowEdge(
+                fromKind: EdgeEndpointKind.step,
+                fromId: ref.source,
+                fromField: ref.field,
+                toKind: EdgeEndpointKind.step,
+                toId: step.id,
+                toField: entry.key,
+              ),
+            );
+          }
+        }
+      }
+    }
+    for (final entry in outputs.entries) {
+      for (final ref in _scanRefs(entry.value)) {
+        if (ref.source == 'inputs' && inputs.containsKey(ref.field)) {
+          result.add(
+            FlowEdge(
+              fromKind: EdgeEndpointKind.inputs,
+              fromId: 'inputs',
+              fromField: ref.field,
+              toKind: EdgeEndpointKind.outputs,
+              toId: 'outputs',
+              toField: entry.key,
+            ),
+          );
+        } else if (stepIds.contains(ref.source)) {
+          result.add(
+            FlowEdge(
+              fromKind: EdgeEndpointKind.step,
+              fromId: ref.source,
+              fromField: ref.field,
+              toKind: EdgeEndpointKind.outputs,
+              toId: 'outputs',
+              toField: entry.key,
+            ),
+          );
+        }
+      }
+    }
+    return result;
+  }
+
+  /// Immutable update — returns a new graph with one step
+  /// replaced. Used by the properties panel + drag handlers.
+  FlowGraph withStepUpdated(String stepId, FlowStep replacement) {
+    final updated = [
+      for (final s in steps)
+        if (s.id == stepId) replacement else s,
+    ];
+    return FlowGraph(
+      name: name,
+      inputs: inputs,
+      steps: updated,
+      outputs: outputs,
+      leadingComment: leadingComment,
+    );
+  }
+
+  /// Append a fresh step at the end of the list.
+  FlowGraph withStepAdded(FlowStep step) {
+    return FlowGraph(
+      name: name,
+      inputs: inputs,
+      steps: [...steps, step],
+      outputs: outputs,
+      leadingComment: leadingComment,
+    );
+  }
+
+  /// Remove the step with [stepId]. Edges pointing to or
+  /// from it become dangling references — the YAML still
+  /// emits them; the engine will fail at run time with a
+  /// clear "unknown ref" error. That's intentional: silently
+  /// rewriting downstream `with:` values would surprise the
+  /// operator.
+  FlowGraph withStepRemoved(String stepId) {
+    return FlowGraph(
+      name: name,
+      inputs: inputs,
+      steps: steps.where((s) => s.id != stepId).toList(),
+      outputs: outputs,
+      leadingComment: leadingComment,
+    );
+  }
+
+  // --- internals ---
+
+  static Map<String, FlowInput> _parseInputs(dynamic node) {
+    if (node is! Map) return const {};
+    final result = <String, FlowInput>{};
+    for (final entry in node.entries) {
+      final key = entry.key.toString();
+      final value = entry.value;
+      if (value is String) {
+        // shorthand: `name: text`
+        result[key] = FlowInput(type: value);
+      } else if (value is Map) {
+        result[key] = FlowInput(
+          type: value['type']?.toString() ?? 'text',
+          defaultValue: value['default']?.toString(),
+          hint: value['hint']?.toString(),
+        );
+      }
+    }
+    return result;
+  }
+
+  static List<FlowStep> _parseSteps(dynamic node) {
+    if (node is! List) return const [];
+    final result = <FlowStep>[];
+    for (final item in node) {
+      if (item is! Map) continue;
+      final id = item['id']?.toString();
+      final use = item['use']?.toString();
+      if (id == null || use == null) continue;
+      final with_ = <String, dynamic>{};
+      final withNode = item['with'];
+      if (withNode is Map) {
+        for (final w in withNode.entries) {
+          with_[w.key.toString()] = w.value;
+        }
+      }
+      result.add(FlowStep(id: id, use: use, with_: with_));
+    }
+    return result;
+  }
+
+  static Map<String, String> _parseOutputs(dynamic node) {
+    if (node is! Map) return const {};
+    final result = <String, String>{};
+    for (final entry in node.entries) {
+      result[entry.key.toString()] = entry.value.toString();
+    }
+    return result;
+  }
+
+  static String _captureLeadingComments(String text) {
+    final lines = text.split('\n');
+    final buf = StringBuffer();
+    for (final line in lines) {
+      final trimmed = line.trimLeft();
+      if (trimmed.isEmpty || trimmed.startsWith('#')) {
+        buf.writeln(line);
+      } else {
+        break;
+      }
+    }
+    return buf.toString();
+  }
+
+  /// Decide whether a value needs quoting in our emitter.
+  /// Plain alphanumerics + simple tokens stay unquoted; the
+  /// moment we see anything YAML could mis-interpret (colon,
+  /// dash at start, quotes, multi-line) we double-quote with
+  /// minimal escaping. Errs on the side of more quoting.
+  static String _scalar(String v) {
+    if (v.isEmpty) return '""';
+    if (v.contains('\n')) {
+      // Use literal block style for multi-line values; keeps
+      // the prompt-heavy flows readable.
+      final indented = v.split('\n').map((l) => '        $l').join('\n');
+      return '|-\n$indented';
+    }
+    final needsQuote =
+        v.contains(':') ||
+        v.contains('#') ||
+        v.startsWith('-') ||
+        v.startsWith('?') ||
+        v.startsWith('[') ||
+        v.startsWith('{') ||
+        v.startsWith('!') ||
+        v.startsWith('&') ||
+        v.startsWith('*') ||
+        v.contains('"') ||
+        v.trim() != v;
+    if (!needsQuote) return v;
+    final escaped = v.replaceAll(r'\', r'\\').replaceAll('"', r'\"');
+    return '"$escaped"';
+  }
+
+  static Iterable<_Ref> _scanRefs(String value) sync* {
+    // F∆I expression syntax: $<step_or_inputs>.<field>
+    // Step IDs and field names match identifier rules:
+    // [A-Za-z_][A-Za-z0-9_-]*. The hub also accepts
+    // `${{ ... }}` but the canonical short form is what every
+    // shipped example uses, so the graph view only mints the
+    // short form on emit. Both are recognised on parse.
+    final shortPattern = RegExp(
+      r'\$([A-Za-z_][A-Za-z0-9_-]*)\.([A-Za-z_][A-Za-z0-9_-]*)',
+    );
+    for (final match in shortPattern.allMatches(value)) {
+      yield _Ref(source: match.group(1)!, field: match.group(2)!);
+    }
+    final longPattern = RegExp(
+      r'\$\{\{\s*([A-Za-z_][A-Za-z0-9_-]*)\.([A-Za-z_][A-Za-z0-9_-]*)\s*\}\}',
+    );
+    for (final match in longPattern.allMatches(value)) {
+      yield _Ref(source: match.group(1)!, field: match.group(2)!);
+    }
+  }
+}
+
+class FlowInput {
+  final String type;
+  final String? defaultValue;
+  final String? hint;
+  const FlowInput({required this.type, this.defaultValue, this.hint});
+  bool get hasOnlyType => defaultValue == null && hint == null;
+}
+
+class FlowStep {
+  final String id;
+  final String use;
+  final Map<String, dynamic> with_;
+
+  const FlowStep({required this.id, required this.use, this.with_ = const {}});
+
+  /// True when the step is the built-in approval gate. The
+  /// graph view styles these differently so the operator can
+  /// see at a glance where the flow waits for a human.
+  bool get isApproval => use.startsWith('system.approval@');
+
+  FlowStep copyWith({String? id, String? use, Map<String, dynamic>? with_}) {
+    return FlowStep(
+      id: id ?? this.id,
+      use: use ?? this.use,
+      with_: with_ ?? this.with_,
+    );
+  }
+}
+
+enum EdgeEndpointKind { inputs, step, outputs }
+
+/// One arrow on the graph. Identifies which step/field it
+/// leaves from and which step/field it enters.
+class FlowEdge {
+  final EdgeEndpointKind fromKind;
+  final String fromId;
+  final String fromField;
+  final EdgeEndpointKind toKind;
+  final String toId;
+  final String toField;
+
+  const FlowEdge({
+    required this.fromKind,
+    required this.fromId,
+    required this.fromField,
+    required this.toKind,
+    required this.toId,
+    required this.toField,
+  });
+}
+
+class _Ref {
+  final String source;
+  final String field;
+  const _Ref({required this.source, required this.field});
+}

lib/src/model/layout_store.dart

@@ -0,0 +1,144 @@
+// Layout sidecar — node positions persisted alongside a flow.
+//
+// Position metadata is intentionally kept OUT of the flow
+// YAML. Reasons:
+//
+//   1. The YAML is what `fai run` consumes, what gets
+//      tamper-checked against the audit log, and what shows
+//      up in pull requests. It should describe behaviour
+//      only, not where a node happened to be on a screen.
+//
+//   2. Two operators viewing the same flow may legitimately
+//      want different layouts. The sidecar is per-user (and
+//      can be re-laid-out fresh from any host).
+//
+//   3. If the sidecar is missing or corrupt, the graph view
+//      falls back to a deterministic auto-layout. The flow
+//      itself never breaks.
+//
+// Storage layout:
+//
+//   ~/.fai/data/flows/.layout/<flow-name>.json
+//
+// One file per flow. JSON shape:
+//
+//   {
+//     "version": 1,
+//     "positions": {
+//       "<step-id>": { "x": 120.0, "y": 80.0 }
+//     }
+//   }
+
+import 'dart:convert';
+import 'dart:io';
+
+class FlowLayout {
+  final Map<String, NodePosition> positions;
+  const FlowLayout({this.positions = const {}});
+
+  factory FlowLayout.empty() => const FlowLayout();
+
+  Map<String, dynamic> toJson() => {
+    'version': 1,
+    'positions': {
+      for (final entry in positions.entries)
+        entry.key: {'x': entry.value.x, 'y': entry.value.y},
+    },
+  };
+
+  factory FlowLayout.fromJson(Map<String, dynamic> json) {
+    final raw = json['positions'];
+    if (raw is! Map) return FlowLayout.empty();
+    final positions = <String, NodePosition>{};
+    for (final entry in raw.entries) {
+      final v = entry.value;
+      if (v is! Map) continue;
+      final x = (v['x'] as num?)?.toDouble();
+      final y = (v['y'] as num?)?.toDouble();
+      if (x == null || y == null) continue;
+      positions[entry.key.toString()] = NodePosition(x, y);
+    }
+    return FlowLayout(positions: positions);
+  }
+
+  FlowLayout withPosition(String stepId, NodePosition pos) {
+    return FlowLayout(positions: {...positions, stepId: pos});
+  }
+
+  FlowLayout withoutStep(String stepId) {
+    return FlowLayout(
+      positions: {
+        for (final entry in positions.entries)
+          if (entry.key != stepId) entry.key: entry.value,
+      },
+    );
+  }
+}
+
+class NodePosition {
+  final double x;
+  final double y;
+  const NodePosition(this.x, this.y);
+
+  @override
+  bool operator ==(Object other) =>
+      other is NodePosition && other.x == x && other.y == y;
+
+  @override
+  int get hashCode => Object.hash(x, y);
+}
+
+/// On-disk persistence for [FlowLayout]. Synchronous I/O is
+/// fine — these files are tiny (a few KB at most) and the
+/// graph view loads / saves them at well-defined moments
+/// (open file, drag end), not in the render loop.
+class LayoutStore {
+  /// Directory `~/.fai/data/flows/.layout/`, created on first
+  /// write. Hidden so it doesn't show up in `fai admin
+  /// flows list` style enumerations.
+  static String defaultDir() {
+    final home =
+        Platform.environment['HOME'] ??
+        Platform.environment['USERPROFILE'] ??
+        '.';
+    return '$home/.fai/data/flows/.layout';
+  }
+
+  /// Load the layout for [flowName]. Returns an empty layout
+  /// if the sidecar doesn't exist yet (first time the flow is
+  /// opened in the graph view), or if it fails to parse.
+  static FlowLayout loadSync(String flowName) {
+    try {
+      final file = File('${defaultDir()}/$flowName.json');
+      if (!file.existsSync()) return FlowLayout.empty();
+      final raw = file.readAsStringSync();
+      final json = jsonDecode(raw);
+      if (json is! Map<String, dynamic>) return FlowLayout.empty();
+      return FlowLayout.fromJson(json);
+    } catch (_) {
+      return FlowLayout.empty();
+    }
+  }
+
+  /// Persist [layout] for [flowName]. Atomic via write-rename
+  /// so a crash mid-write leaves the previous good copy
+  /// intact.
+  static void saveSync(String flowName, FlowLayout layout) {
+    final dir = Directory(defaultDir());
+    if (!dir.existsSync()) dir.createSync(recursive: true);
+    final final_ = File('${dir.path}/$flowName.json');
+    final tmp = File('${dir.path}/$flowName.json.tmp');
+    tmp.writeAsStringSync(
+      const JsonEncoder.withIndent('  ').convert(layout.toJson()),
+      flush: true,
+    );
+    tmp.renameSync(final_.path);
+  }
+
+  /// Remove the sidecar — call this when the operator
+  /// deletes a flow so we don't leak stale layouts.
+  static void deleteSync(String flowName) {
+    final file = File('${defaultDir()}/$flowName.json');
+    if (file.existsSync()) file.deleteSync();
+  }
+}