fix Claude pane hang: cap tail, parse off-isolate, coalesce notifies

The native Claude pane (T-137) picks the newest transcript JSONL by
mtime — which on a live session is this multi-MB active file. On attach
it read and parsed the whole file synchronously on the UI isolate and
fired notifyListeners per item, freezing the app.

Three fixes: cap the initial read to a recent tail (256KB, injectable
for tests); run JSON parsing in a background isolate via Isolate.run;
coalesce the controller's notifications with a zero-duration Timer so a
burst collapses into one rebuild (a microtask-scheduled notify wouldn't
— stream events deliver one per microtask and the notify interleaves).

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

test/builtin/claude/conversation_view_test.dart

@@ -56,11 +56,15 @@ void main() {
       expect(c.isEmpty, isTrue);
       ctrl.add(_user('hi'));
       ctrl.add(_asst('hello'));
-      await Future<void>.delayed(Duration.zero);
+      // Wait past the coalescing timer (zero-duration, fires after the
+      // microtask queue drains).
+      await Future<void>.delayed(const Duration(milliseconds: 20));
 
       expect(c.items, hasLength(2));
       expect(c.items.first, isA<UserMessage>());
-      expect(notifications, 2);
+      // Notifications are coalesced: a burst of items collapses to a
+      // single notify so the view rebuilds once, not per item.
+      expect(notifications, 1);
       await ctrl.close();
     });