feat(editor): graph-pulse on errors + store-aware install / add-source

Three connected improvements to the analyzer-driven diagnostics:

  - **Pulsing halo on broken graph nodes**. Every step / pseudo-
    node (inputs / outputs) carrying an analyzer issue now
    breathes a red (error) or amber (warning) halo on the graph
    tab — operator sees the problem on the canvas without
    flipping to text. Honours prefers-reduced-motion: reduce-
    motion users get a static halo at the same intensity. The
    canvas reads severity via a new `stepIssueSeverity` map on
    FlowEditorController; pseudo-nodes use `__inputs__` /
    `__outputs__` sentinel ids.

  - **Store-aware install button**. The analyzer now takes a
    second closure, `storeCapabilities`, listing what the public
    store can install. Unknown-capability issues only carry the
    "Install …" quick-fix when the bare cap is in that list;
    otherwise the issue carries an "Add source for …" fix
    instead. Resolves the asymmetry the operator reported: the
    Store didn't show `htw.digiscout/onet.lookup` but the editor
    happily offered to install it (and would have failed). The
    install path no longer lies about itself.

  - **Did-you-mean suggestion**. When the unknown cap is within
    edit-distance two of an installed or store cap (different
    spelling — distance-0 stays hidden because that's an install
    case, not a typo), the analyzer emits a `ReplaceLineValueFix`
    suggesting the closest match. Preserves the version
    constraint by reusing the installed spec when present
    (e.g. `text.echi@^0.1` → `text.echo@^0.1`).

New public surface:

  - `AddModuleSourceFix` + `AddModuleSourceCallback`
  - `FlowEditorPage.storeCapabilities` + `onAddModuleSource`
  - `FlowAnalyzer.stepSeverity` + the `kInputsNodeId` /
    `kOutputsNodeId` sentinels
  - `FlowIssueSeverity` enum + `FlowNode.issueSeverity`

Tests:
  - Install fix only when in store
  - AddModuleSourceFix as fallback when not in store
  - Did-you-mean replaces install when a near miss exists
  - stepSeverity populated for both step ids and pseudo-nodes

All 36 editor tests green. Bumped to 0.18.0.

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

lib/src/widgets/flow_canvas.dart

@@ -33,6 +33,7 @@
 //                         InteractiveViewer.
 
 import 'package:flutter/material.dart';
+import 'package:flutter_code_editor/flutter_code_editor.dart' show IssueType;
 
 import '../editor_controller.dart';
 import '../editor_style.dart';
@@ -875,6 +876,13 @@ class _FlowCanvasState extends State<FlowCanvas>
       outputLabels.length,
     );
     final cardWidth = _stepWidth(step);
+    // Analyzer severity for this step — if its YAML carries an
+    // unknown-capability error or a typed-input warning, we
+    // tag the node so FlowNode renders a pulsing halo. The
+    // running pulse takes priority when both are active; the
+    // node never combines run-glow + error-halo on the same
+    // surface (would mush the colour).
+    final issueSeverity = _issueSeverityFor(step.id);
     return Positioned(
       left: pos.x,
       top: pos.y,
@@ -891,14 +899,12 @@ class _FlowCanvasState extends State<FlowCanvas>
         selected: selected,
         status: status,
         elevated: _style.nodeShadows,
-        // Breathing-pulse on running steps when the active
-        // style allows flow animation. The canvas's existing
-        // _flowAnim ticks 0..1 during runs and is gated on
-        // the same accessibility / style preferences, so we
-        // can route it straight through.
         pulse: _style.flowAnimation && status == FlowNodeStatus.running
             ? _flowAnim
             : null,
+        issueSeverity: status == FlowNodeStatus.running
+            ? null
+            : issueSeverity,
         onTap: () => widget.controller.selectStep(step.id),
         onDrag: (delta) => _applyDrag(step.id, pos, delta, cardHeight),
         onContextMenu: (globalPos) => _showStepContextMenu(step, globalPos),
@@ -930,10 +936,11 @@ class _FlowCanvasState extends State<FlowCanvas>
         portSide: portSide,
         inputPortLabels: labels,
         selected: selected,
-        // Endpoints are selectable too — selecting opens
-        // the inputs / outputs editor in the properties
-        // panel so the operator can rename, retype, or add
-        // entries graphically instead of editing YAML.
+        // Endpoints carry the same issue-halo treatment as
+        // step nodes — the analyzer uses `__inputs__` /
+        // `__outputs__` sentinel ids so an `Unknown input type`
+        // warning lights up the matching endpoint card.
+        issueSeverity: _issueSeverityFor(nodeId),
         onTap: () => widget.controller.selectStep(nodeId),
         onDrag: (delta) => _applyDrag(
           nodeId,
@@ -945,6 +952,18 @@ class _FlowCanvasState extends State<FlowCanvas>
     );
   }
 
+  /// Map the controller's per-step issue severity (from the
+  /// YAML analyzer) onto the FlowNode's enum. Returns null
+  /// when the step has no issues so the node renders normally.
+  FlowIssueSeverity? _issueSeverityFor(String stepId) {
+    final raw = widget.controller.stepIssueSeverity[stepId];
+    return switch (raw) {
+      IssueType.error => FlowIssueSeverity.error,
+      IssueType.warning => FlowIssueSeverity.warning,
+      _ => null,
+    };
+  }
+
   /// Single drag entry point used by every node on the
   /// canvas. Converts a screen-space delta to canvas-space
   /// (via the current InteractiveViewer scale), clamps the