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_node.dart

@@ -154,6 +154,12 @@ enum NodeVisualKind { module, approval, inputs, outputs }
 ///              endpoint OUT to downstream steps).
 enum NodePortSide { left, right }
 
+/// Severity tag passed to [FlowNode.issueSeverity]. Mirrors
+/// flutter_code_editor's `IssueType` but lives in the editor
+/// package so flow_node.dart doesn't pull the code-editor
+/// transitively into widget tests that don't need it.
+enum FlowIssueSeverity { error, warning }
+
 class FlowNode extends StatelessWidget {
   final String id;
   final String title;
@@ -224,6 +230,14 @@ class FlowNode extends StatelessWidget {
   /// no pulse is rendered.
   final Listenable? pulse;
 
+  /// When non-null, the node renders a permanently pulsing
+  /// halo in the appropriate severity colour — red for an
+  /// analyzer-flagged error, amber for a warning — so the
+  /// operator scanning the canvas immediately sees which step
+  /// the YAML linter is unhappy about, without switching to
+  /// the text tab.
+  final FlowIssueSeverity? issueSeverity;
+
   const FlowNode({
     super.key,
     required this.id,
@@ -244,6 +258,7 @@ class FlowNode extends StatelessWidget {
     this.onContextMenu,
     this.elevated = true,
     this.pulse,
+    this.issueSeverity,
     this.width = NodeGeometry.width,
   });
 
@@ -268,10 +283,6 @@ class FlowNode extends StatelessWidget {
         child: AnimatedBuilder(
           animation: pulse!,
           builder: (context, child) {
-            // Derive a 0..1 breath value from the controller.
-            // Use a sine wave so the glow eases in + out
-            // rather than ramping linearly — reads as
-            // "breathing" instead of "flickering".
             final t = (pulse! is Animation<double>)
                 ? (pulse! as Animation<double>).value
                 : 0.0;
@@ -294,6 +305,19 @@ class FlowNode extends StatelessWidget {
         ),
       );
     }
+    if (issueSeverity != null) {
+      // Always-on breathing halo in the severity colour. Owns
+      // its own AnimationController so it ticks even when no
+      // step is running (no shared pulse listenable upstream).
+      return SizedBox(
+        width: width,
+        height: height,
+        child: _IssueHalo(
+          severity: issueSeverity!,
+          child: cardWidget,
+        ),
+      );
+    }
     return SizedBox(width: width, height: height, child: cardWidget);
   }
 
@@ -592,3 +616,80 @@ NodeVisualKind kindForStep(FlowStep step) {
   if (step.isApproval) return NodeVisualKind.approval;
   return NodeVisualKind.module;
 }
+
+/// Permanently breathing halo painted around a node carrying
+/// an analyzer issue. Owns its own AnimationController so it
+/// ticks regardless of the canvas's run-state. Halo colour
+/// matches severity — error red or warning amber. Honours
+/// `MediaQuery.disableAnimations` so reduce-motion operators
+/// still get a static halo without the breathing oscillation.
+class _IssueHalo extends StatefulWidget {
+  final FlowIssueSeverity severity;
+  final Widget child;
+  const _IssueHalo({required this.severity, required this.child});
+
+  @override
+  State<_IssueHalo> createState() => _IssueHaloState();
+}
+
+class _IssueHaloState extends State<_IssueHalo>
+    with SingleTickerProviderStateMixin {
+  late final AnimationController _controller = AnimationController(
+    vsync: this,
+    duration: const Duration(milliseconds: 1600),
+  )..repeat();
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
+
+  Color _tone(BuildContext context) {
+    final theme = Theme.of(context);
+    return widget.severity == FlowIssueSeverity.error
+        ? theme.colorScheme.error
+        : const Color(0xFFEF6C00);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final tone = _tone(context);
+    final reduceMotion = MediaQuery.disableAnimationsOf(context);
+    if (reduceMotion) {
+      return DecoratedBox(
+        decoration: BoxDecoration(
+          borderRadius: BorderRadius.circular(FaiRadius.md),
+          boxShadow: [
+            BoxShadow(
+              color: tone.withValues(alpha: 0.35),
+              blurRadius: 18,
+              spreadRadius: 1.5,
+            ),
+          ],
+        ),
+        child: widget.child,
+      );
+    }
+    return AnimatedBuilder(
+      animation: _controller,
+      builder: (context, child) {
+        final breath = 0.5 + 0.5 * math.sin(_controller.value * 2 * math.pi);
+        return DecoratedBox(
+          decoration: BoxDecoration(
+            borderRadius: BorderRadius.circular(FaiRadius.md),
+            boxShadow: [
+              BoxShadow(
+                color: tone.withValues(alpha: 0.22 + 0.30 * breath),
+                blurRadius: 16 + 8 * breath,
+                spreadRadius: 0.5 + 2.0 * breath,
+              ),
+            ],
+          ),
+          child: child,
+        );
+      },
+      child: widget.child,
+    );
+  }
+}