add ClidePane primitive + focus-driven status-bar slot

Replace the MessageBus-based pane-context slot with a focus-driven one.
Panes keep their status widget locally; the FocusTracker holds the
focused pane's widget (activeStatusWidget) and ClidePane conveys it to
the shared slot only while its contribution is focused, re-conveying on
change and clearing on blur. The status-bar item just renders
focus.activeStatusWidget, height-clamped and marquee-scrolled when it
overflows. Removes the publish/subscribe race the bus version had.

T-150.

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

test/kernel/src/focus_test.dart

@@ -0,0 +1,54 @@
+/// Tests for FocusTracker's status-widget slot (T-150).
+library;
+
+import 'package:clide/kernel/src/focus.dart';
+import 'package:clide/kernel/src/panels/slot_id.dart';
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  group('FocusTracker status widget', () {
+    test('setStatusWidget applies only for the focused contribution', () {
+      final f = FocusTracker();
+      f.setActive(slot: Slots.workspace, contributionId: 'a');
+      const wa = SizedBox(key: ValueKey('a'));
+      f.setStatusWidget('a', wa);
+      expect(f.activeStatusWidget, wa);
+
+      // A background contribution's update is ignored.
+      f.setStatusWidget('b', const SizedBox(key: ValueKey('b')));
+      expect(f.activeStatusWidget, wa);
+    });
+
+    test('a focus change clears the previous status widget', () {
+      final f = FocusTracker();
+      f.setActive(slot: Slots.workspace, contributionId: 'a');
+      f.setStatusWidget('a', const SizedBox());
+      expect(f.activeStatusWidget, isNotNull);
+
+      f.setActive(slot: Slots.workspace, contributionId: 'b');
+      expect(f.activeStatusWidget, isNull);
+    });
+
+    test('clear() resets focus and status', () {
+      final f = FocusTracker();
+      f.setActive(slot: Slots.workspace, contributionId: 'a');
+      f.setStatusWidget('a', const SizedBox());
+      f.clear();
+      expect(f.activeContributionId, isNull);
+      expect(f.activeStatusWidget, isNull);
+    });
+
+    test('setStatusWidget notifies on change, not on a repeat', () {
+      final f = FocusTracker();
+      f.setActive(slot: Slots.workspace, contributionId: 'a');
+      var n = 0;
+      f.addListener(() => n++);
+      const w = SizedBox();
+      f.setStatusWidget('a', w);
+      expect(n, 1);
+      f.setStatusWidget('a', w); // identical → no notify
+      expect(n, 1);
+    });
+  });
+}