open a file in the diff panel via clide ui open diff

Adds diff as a fourth ui.open target. The diff extension now retains an
app-scoped DiffController and subscribes to its builtin.diff/selection
channel: a selection reveals the diff tab and focuses the file, which
the view scrolls into view and highlights. Retaining the controller in
the extension (not the view) lets a focus survive the tab being
revealed/remounted, mirroring the ReaderNav viewers.

Closes T-233.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/builtin/diff/src/extension.dart

@@ -1,3 +1,6 @@
+import 'dart:async';
+
+import 'package:clide/builtin/diff/src/diff_controller.dart';
 import 'package:clide/builtin/diff/src/diff_view.dart';
 import 'package:clide/extension/extension.dart';
 import 'package:clide/kernel/kernel.dart';
@@ -8,10 +11,17 @@ class DiffExtension extends ClideExtension {
   @override
   String get title => 'Diff';
   @override
-  String get version => '0.1.0';
+  String get version => '0.2.0';
   @override
   List<String> get dependsOn => const [];
 
+  /// App-scoped controller retained across tab reveal/remount so a
+  /// `ui open diff <path>` focus survives the view being (re)built (T-233).
+  /// Built in [activate] where the kernel ipc/events are in scope; the view
+  /// renders it but does not own it.
+  DiffController? _controller;
+  StreamSubscription<Message>? _sub;
+
   @override
   List<ContributionPoint> get contributions => [
         TabContribution(
@@ -21,7 +31,29 @@ class DiffExtension extends ClideExtension {
           titleKey: 'tab.title',
           i18nNamespace: id,
           priority: -70,
-          build: (_) => const DiffView(),
+          build: (_) => DiffView(controller: _controller),
         ),
       ];
+
+  @override
+  Future<void> activate(ClideExtensionContext ctx) async {
+    _controller = DiffController(ipc: ctx.ipc, events: ctx.events)..load();
+    // `clide ui open diff <path>` publishes a 'selection' here (T-233, the
+    // diff-panel arm of D-6 parity). Reveal the diff tab and focus the file —
+    // the same reveal mechanism the ReaderNav viewers use for their tabs.
+    _sub = ctx.messages.subscribe(publisher: id, channel: 'selection').listen((msg) {
+      final path = msg.data['path'];
+      if (path is! String || path.isEmpty) return;
+      ctx.panels.activateTab(Slots.workspace, 'diff.view');
+      _controller?.focus(path);
+    });
+  }
+
+  @override
+  Future<void> deactivate() async {
+    await _sub?.cancel();
+    _sub = null;
+    _controller?.dispose();
+    _controller = null;
+  }
 }