test sweep: cover kernel/src/{events,ipc} (T-91)

Three new test files + a small DaemonClient dispose-safety fix:

- test/kernel/src/events/types_test.dart (7 tests): every ClideEvent
  subclass's subsystem / kind / payload contract + the
  ClideEventEnvelope v1 JSON shape.
- test/kernel/src/events/message_bus_test.dart (6 tests): Message
  shape, MessageBus publish/subscribe/dispose, filter-by-publisher,
  filter-by-channel, intersection.
- test/kernel/src/ipc/client_test.dart (9 tests): real Unix-socket
  roundtrip via a _TestDaemon helper — connect + correlate request/
  response, event forwarding to the DaemonBus, malformed-line skip,
  daemon-disconnect failing pending requests, stop cleanup, dispose,
  connect-failure-then-reconnect, daemon-sent-Request warn-and-skip,
  DaemonConnectionChanged emission.

Fix in lib/kernel/src/ipc/client.dart: _setConnected now skips
notifyListeners / event emit when _disposed. The socket stream's
onDone can fire after dispose runs, which previously hit
ChangeNotifier's "used after disposed" assertion. State flip stays
unconditional so stop()'s explicit transition still works.

Coverage: ipc/client.dart 14% -> 92% (79/86; remaining 7 lines are
the socket onError callback + 1 const ctor phantom); events/types
.dart 95% (37/39 — 2 const-ctor phantoms); events/message_bus.dart
100%; events/bus.dart stays 100%.

Total coverage 71.93% -> 73.34%; floor bumped to 73.

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

lib/kernel/src/ipc/client.dart

@@ -149,6 +149,10 @@ class DaemonClient extends ChangeNotifier {
   void _setConnected(bool v) {
     if (_connected == v) return;
     _connected = v;
+    // Skip side-effects (event emit + notifyListeners) after dispose —
+    // the socket stream's onDone can fire post-dispose and would
+    // otherwise hit ChangeNotifier's "used after disposed" assert.
+    if (_disposed) return;
     _events.emit(DaemonConnectionChanged(connected: v));
     notifyListeners();
   }