feat(canvas): open .canvas files in a workspace pane (T-322)

Makes the canvas foundation (parser/renderer/view) reachable. The
extension owns an app-scoped MultitabController (the diff/T-233
pattern) so open documents survive the pane being rebuilt; each
document is a real sub-tab per the refinement decision, kept alive
across switches. Routing goes through the existing seams instead of
the dead TabContribution.fileGlobs field: openWorkspaceFile gains a
.canvas branch mirroring .md, and ui.open gains a canvas reader for
D-6 parity (clide ui open canvas <path>). Also corrects the
json_canvas doc header that claimed SVG-lowering — the interactive
pane paints the model directly (D-103 live-widget exception).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/kernel/file_open_test.dart

@@ -0,0 +1,70 @@
+/// openWorkspaceFile (T-51/T-187) — the single dispatch point for file
+/// activations: `.md` → markdown reader, `.canvas` → canvas pane (T-322),
+/// everything else → the editor via IPC. Also records recents.
+library;
+
+import 'package:clide/clide.dart';
+import 'package:clide/kernel/kernel.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import '../helpers/kernel_fixture.dart';
+
+void main() {
+  late KernelFixture f;
+  late List<Message> published;
+  late List<Object?> editorOpens;
+
+  setUp(() async {
+    f = await KernelFixture.create();
+    published = [];
+    editorOpens = [];
+    f.services.messages.subscribe().listen(published.add);
+    f.ipc.stub('editor.open', (args) async {
+      editorOpens.add(args['path']);
+      return IpcResponse.ok(id: '1', data: const {});
+    });
+  });
+  tearDown(() => f.dispose());
+
+  Future<void> deliver() => Future<void>.delayed(Duration.zero);
+
+  test('.md routes to the markdown reader via the bus', () async {
+    openWorkspaceFile(f.services, 'docs/plan.md');
+    await deliver();
+    expect(published, hasLength(1));
+    expect(published.single.publisher, 'builtin.markdown');
+    expect(published.single.channel, 'selection');
+    expect(published.single.data, {'path': 'docs/plan.md'});
+    expect(editorOpens, isEmpty);
+  });
+
+  test('.canvas routes to the canvas pane via the bus (T-322)', () async {
+    openWorkspaceFile(f.services, 'notes/board.canvas');
+    await deliver();
+    expect(published, hasLength(1));
+    expect(published.single.publisher, 'builtin.canvas');
+    expect(published.single.channel, 'selection');
+    expect(published.single.data, {'path': 'notes/board.canvas'});
+    expect(editorOpens, isEmpty);
+  });
+
+  test('extension match is case-insensitive', () async {
+    openWorkspaceFile(f.services, 'BOARD.CANVAS');
+    await deliver();
+    expect(published.single.publisher, 'builtin.canvas');
+  });
+
+  test('everything else opens in the editor over IPC', () async {
+    openWorkspaceFile(f.services, 'lib/main.dart');
+    await deliver();
+    expect(published, isEmpty);
+    expect(editorOpens, ['lib/main.dart']);
+  });
+
+  test('records the open in recent files; empty path is a no-op', () async {
+    openWorkspaceFile(f.services, 'notes/board.canvas');
+    openWorkspaceFile(f.services, '');
+    await deliver();
+    expect(f.services.recentFiles.paths, ['notes/board.canvas']);
+  });
+}