split EventBus into DaemonBus + MessageBus

DaemonBus (was EventBus): typed events for system/IPC layer.
MessageBus: channel-based pub/sub for UI/extension coordination.
Messages carry publisher (auto-stamped from extension ID),
channel (required), timestamp, and payload. Subscribe by
publisher, channel, or both — zero collision across extensions.

Extension context gains publish() and subscribe() convenience
methods that auto-stamp the extension's ID as publisher.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/kernel/kernel.dart

@@ -12,6 +12,7 @@
 library;
 
 export 'src/events/bus.dart';
+export 'src/events/message_bus.dart';
 export 'src/events/types.dart';
 export 'src/ipc/client.dart';
 export 'src/log.dart';