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/widgets/src/clide_marquee_test.dart

@@ -0,0 +1,32 @@
+/// Tests for ClideMarquee (T-150): static when the child fits, scrolls
+/// (looped copy) when it overflows; clips its box.
+library;
+
+import 'package:clide/widgets/widgets.dart';
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+Widget _boxed(double width, Widget child) => Directionality(
+      textDirection: TextDirection.ltr,
+      child: Center(
+        child: SizedBox(width: width, height: 16, child: child),
+      ),
+    );
+
+void main() {
+  testWidgets('shows the child statically when it fits', (tester) async {
+    await tester.pumpWidget(_boxed(300, const ClideMarquee(child: Text('short'))));
+    await tester.pump();
+    expect(find.text('short'), findsOneWidget);
+    await tester.pumpWidget(const SizedBox()); // tear down the ticker
+  });
+
+  testWidgets('renders a looped copy and runs without overflow when wider than the slot', (tester) async {
+    await tester.pumpWidget(_boxed(40, const ClideMarquee(child: Text('a long status line that overflows the slot'))));
+    await tester.pump(); // measure
+    await tester.pump(const Duration(milliseconds: 100)); // advance the ticker
+    expect(find.text('a long status line that overflows the slot'), findsWidgets);
+    expect(tester.takeException(), isNull);
+    await tester.pumpWidget(const SizedBox()); // dispose → stop ticker
+  });
+}