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/quick_fix.dart

@@ -50,7 +50,7 @@ class ReplaceLineValueFix extends QuickFix {
 }
 
 /// Ask the host (Studio) to install the named capability via
-/// the Hub. The editor delegates to the
+/// the Hub's store-backed install. The editor delegates to the
 /// [InstallCapabilityCallback] passed into `FlowEditorPage`
 /// and reanalyzes the document once the host returns.
 @immutable
@@ -66,6 +66,28 @@ class InstallCapabilityFix extends QuickFix {
   }) : super(label);
 }
 
+/// Ask the host to register a new module source for an unknown
+/// capability — used when the capability isn't in the public
+/// store. The host's handler typically prompts the operator
+/// for a local path (`fai install --link`) or a URL
+/// (`fai install <url>`), then installs and reanalyzes.
+///
+/// This is the recovery path for private modules: the public
+/// store doesn't know about `htw.digiscout/onet-lookup`, but
+/// the operator can point the hub at the local clone.
+@immutable
+class AddModuleSourceFix extends QuickFix {
+  /// The capability the operator wrote — the host uses it to
+  /// pre-fill its prompt ("Where can `<capability>` be
+  /// installed from?").
+  final String capability;
+
+  const AddModuleSourceFix({
+    required this.capability,
+    required String label,
+  }) : super(label);
+}
+
 /// Host-side install handler signature. Returns the new
 /// installed-capability list after the install completes (used
 /// by the editor to refresh its analyzer without round-tripping
@@ -75,6 +97,13 @@ class InstallCapabilityFix extends QuickFix {
 typedef InstallCapabilityCallback =
     Future<List<String>?> Function(String capability);
 
+/// Host-side "add module source" handler. Receives the bare
+/// capability spec the operator typed; expected to prompt the
+/// operator for a local path / URL and then install it.
+/// Same return contract as [InstallCapabilityCallback].
+typedef AddModuleSourceCallback =
+    Future<List<String>?> Function(String capability);
+
 /// Hover-tooltip request — the controller emits one of these
 /// via a ValueNotifier when the pointer enters an issue range,
 /// and clears it when the pointer leaves both the range and the