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();
     });
 

test/builtin/claude/transcript_reader_test.dart

@@ -669,6 +669,43 @@ void main() {
       expect(collected, hasLength(1));
       expect((collected.first as UserMessage).text, 'real message');
     });
+
+    test('initial attach reads only the recent tail (initialTailBytes cap)', () async {
+      final projectDir = mungedDir(tempBase, workspace);
+      await projectDir.create(recursive: true);
+      final sessionFile = File('${projectDir.path}/session-abc.jsonl');
+
+      // A long pre-existing transcript — the kind that froze the UI when
+      // parsed in full on attach.
+      writeLines(sessionFile, [
+        for (var i = 0; i < 200; i++) assistantText('a$i', 'reply number $i'),
+      ]);
+
+      // Cap the initial read well below the file size so only the last
+      // records are within the tail window.
+      final reader = TranscriptReader(
+        workspace,
+        projectsBase: tempBase.path,
+        pollInterval: const Duration(milliseconds: 20),
+        initialTailBytes: 256,
+      );
+      final collected = <ConversationItem>[];
+      final sub = reader.stream.listen(collected.add);
+
+      await Future<void>.delayed(const Duration(milliseconds: 200));
+
+      await sub.cancel();
+      await reader.dispose();
+
+      // Far fewer than the 200 records — only the recent tail was read.
+      expect(collected, isNotEmpty);
+      expect(collected.length, lessThan(200));
+      // The most recent record is always intact at the end of the file.
+      expect(
+        collected.whereType<AssistantTextMessage>().last.text,
+        'reply number 199',
+      );
+    });
   });
 }