fix ClidePane notifying focus listeners during build

ClidePane.didChangeDependencies/didUpdateWidget run in the build phase and
called FocusTracker.setStatusWidget -> notifyListeners() synchronously,
rebuilding the focus-listening status-bar item mid-build — Flutter threw
"markNeedsBuild called during build" on every frame once a Claude pane was
focused. The convey now defers to a post-frame callback when mid-build
(re-checking focus then) and applies immediately otherwise. The T-150
widget tests missed this because no focus listener was in their tree;
added a regression test with PaneContextStatusItem present.

T-159.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

test/widgets/src/clide_pane_test.dart

@@ -2,6 +2,7 @@
 /// FocusTracker slot only while it is the shown (focused + active) pane.
 library;
 
+import 'package:clide/builtin/claude/src/pane_context_status.dart';
 import 'package:clide/kernel/src/panels/slot_id.dart';
 import 'package:clide/widgets/widgets.dart';
 import 'package:flutter/widgets.dart';
@@ -36,6 +37,30 @@ void main() {
     expect(focus.activeStatusWidget, isNull); // focus moved away → cleared
   });
 
+  testWidgets('mounting while focused does not notify focus listeners during build', (tester) async {
+    // Regression: ClidePane.didChangeDependencies runs during the build phase,
+    // and conveying synchronously rebuilt the focus-listening status item
+    // mid-build ("markNeedsBuild during build"). A focus listener must be in
+    // the tree to reproduce it.
+    f.services.focus.setActive(slot: Slots.workspace, contributionId: 'pane.x');
+    await tester.pumpWidget(harness(
+      f,
+      const Column(
+        children: [
+          PaneContextStatusItem(),
+          ClidePane(
+            contributionId: 'pane.x',
+            statusWidget: Text('S', textDirection: TextDirection.ltr),
+            child: SizedBox(),
+          ),
+        ],
+      ),
+    ));
+    await tester.pump(); // run the deferred post-frame convey
+    expect(tester.takeException(), isNull);
+    expect(f.services.focus.activeStatusWidget, isA<Text>());
+  });
+
   testWidgets('an inactive pane never conveys', (tester) async {
     await tester.pumpWidget(harness(
       f,