add ClideCollapserCard primitive + shared card metrics (T-305)

First slice of the conversation-panel card unification. Adds the
category-3 collapser primitive (lib/widgets/): a list of 1..N inner item
cards, collapsed ticker <-> expanded framed inner canvas, with the agreed
chrome — color drives the border + chevron/label tint, a fixed-width
counter slot, the status icon hard against the right edge, chevron hard
against the left edge, background + caret toggle (D-78 tail-follow). The
aggregate status/count/echoed-title are caller-computed, so the widget
stays free of conversation semantics; inner items keep their own per-item
status.

Also adds clide_card_metrics.dart — shared spacing constants (gap, radius,
header padding, counter slot width) for all three card categories.

Not yet wired into the stream (no user-visible change); migration of the
group/tool cards follows. Widget test + golden included.

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

test/widgets/src/clide_collapser_card_test.dart

@@ -0,0 +1,139 @@
+/// Widget tests for [ClideCollapserCard] (T-305) — the conversation stream's
+/// category-3 collapsible card. Covers the collapsed ticker chrome (label +
+/// echoed summary + fixed counter + aggregate status), expand/collapse via the
+/// row, the background-as-toggle, item taps not hijacked, the keyboard/AT path,
+/// and the `color` driving border + label tint.
+library;
+
+import 'package:clide/widgets/widgets.dart';
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import '../../helpers/kernel_fixture.dart';
+import '../../helpers/widget_harness.dart';
+
+void main() {
+  late KernelFixture f;
+  setUp(() async => f = await KernelFixture.create());
+  tearDown(() => f.dispose());
+
+  Future<void> pump(
+    WidgetTester tester, {
+    List<Widget>? children,
+    String label = 'Edits',
+    String? summary = 'clide_markdown.dart',
+    String? counter = '3 edits',
+    ClideRunStatus? status = ClideRunStatus.success,
+    Color? color,
+    bool expanded = false,
+  }) async {
+    await tester.pumpWidget(harness(
+      f,
+      Align(
+        alignment: Alignment.topLeft,
+        child: SizedBox(
+          width: 400,
+          child: ClideCollapserCard(
+            label: label,
+            collapsedSummary: summary,
+            counter: counter,
+            status: status,
+            color: color,
+            initiallyExpanded: expanded,
+            children: children ?? const [Text('item body', textDirection: TextDirection.ltr)],
+          ),
+        ),
+      ),
+    ));
+    await tester.pump();
+  }
+
+  testWidgets('collapsed ticker shows label, echoed summary, counter + status; items hidden', (tester) async {
+    await pump(tester);
+    expect(find.text('Edits'), findsOneWidget);
+    expect(find.text('clide_markdown.dart'), findsOneWidget);
+    expect(find.text('3 edits'), findsOneWidget);
+    expect(find.byType(ClideStatusIndicator), findsOneWidget);
+    expect(find.bySemanticsLabel('Edits, 3 edits, collapsed'), findsOneWidget);
+    expect(find.text('item body'), findsNothing);
+  });
+
+  testWidgets('tapping the ticker expands to the framed item canvas', (tester) async {
+    await pump(tester);
+    await tester.tap(find.bySemanticsLabel('Edits, 3 edits, collapsed'));
+    await tester.pump();
+    expect(find.bySemanticsLabel('Edits, 3 edits, expanded'), findsOneWidget);
+    expect(find.text('item body'), findsOneWidget);
+    // The label + chrome persist in the expanded header.
+    expect(find.text('Edits'), findsOneWidget);
+    expect(find.text('3 edits'), findsOneWidget);
+  });
+
+  testWidgets('a single item still sits in the list (1-item collapser)', (tester) async {
+    await pump(tester, counter: '1 edit', children: const [Text('only item', textDirection: TextDirection.ltr)], expanded: true);
+    expect(find.text('only item'), findsOneWidget);
+    expect(find.text('1 edit'), findsOneWidget);
+  });
+
+  testWidgets('a tap on the frame BACKGROUND (gutter) collapses it', (tester) async {
+    await pump(tester, expanded: true);
+    expect(find.text('item body'), findsOneWidget);
+
+    // The left gutter (x = left+4) is collapser background — items start at
+    // left+10 — and below the header, so this hits the background toggle.
+    final r = tester.getRect(find.byType(ClideCollapserCard));
+    await tester.tapAt(Offset(r.left + 4, r.center.dy));
+    await tester.pump();
+    expect(find.text('item body'), findsNothing);
+    expect(find.bySemanticsLabel('Edits, 3 edits, collapsed'), findsOneWidget);
+  });
+
+  testWidgets('a tap on an item does NOT toggle the collapser', (tester) async {
+    await pump(tester, expanded: true);
+    await tester.tap(find.text('item body'));
+    await tester.pump();
+    expect(find.text('item body'), findsOneWidget); // still expanded
+    expect(find.bySemanticsLabel('Edits, 3 edits, expanded'), findsOneWidget);
+  });
+
+  testWidgets('the ticker is keyboard-focusable and toggles on Activate (a11y)', (tester) async {
+    await pump(tester);
+    final focusWidget = tester.widget<Focus>(
+      find.ancestor(of: find.text('clide_markdown.dart'), matching: find.byType(Focus)).first,
+    );
+    focusWidget.focusNode!.requestFocus();
+    await tester.pump();
+    expect(focusWidget.focusNode!.hasFocus, isTrue);
+
+    Actions.invoke(focusWidget.focusNode!.context!, const ActivateIntent());
+    await tester.pump();
+    expect(find.bySemanticsLabel('Edits, 3 edits, expanded'), findsOneWidget);
+  });
+
+  testWidgets('color drives the border + the label tint', (tester) async {
+    const teal = Color(0xFF00AB9A);
+    await pump(tester, color: teal);
+
+    // Label text carries the color.
+    final label = tester.widget<ClideText>(find.byWidgetPredicate((w) => w is ClideText && w.data == 'Edits'));
+    expect(label.color, teal);
+
+    // Some frame Container carries a border in the color.
+    expect(
+      find.byWidgetPredicate((w) {
+        if (w is! Container) return false;
+        final d = w.decoration;
+        return d is BoxDecoration && d.border?.bottom.color == teal || (d is BoxDecoration && d.border is Border && (d.border as Border).top.color == teal);
+      }),
+      findsWidgets,
+    );
+  });
+
+  testWidgets('no counter + no status renders a bare ticker without error', (tester) async {
+    await pump(tester, counter: null, status: null, summary: null);
+    expect(find.text('Edits'), findsOneWidget);
+    expect(find.byType(ClideStatusIndicator), findsNothing);
+    expect(tester.takeException(), isNull);
+    expect(find.bySemanticsLabel('Edits, collapsed'), findsOneWidget);
+  });
+}