add pane subsystem to the daemon + IPC event broadcast

Implements the Tier-1 pane subsystem from D-006: spawn / list / focus /
close / write / resize / tail commands, plus pane.spawned / output /
exit / resized / focused / closed events. PaneRegistry owns per-pane
PtySession lifecycles and id generation (p_N); a DaemonEventSink seam
keeps pane code decoupled from the IPC server package.

DaemonServer.broadcast() fans events out to every connected client.
Per-client subsystem/id filtering (`tail --filter pane:p_7`) is
deferred — Tier 1 broadcasts everything and the subscriber discards.

Panes carry a `kind:` field (terminal | claude). Step 7 (builtin.claude)
adds the claude-specific pane flow on top of this generic substrate —
the subsystem itself stays neutral.

14 new core tests: registry unit coverage (spawn → pane.spawned event,
output → base64 events, write/resize/close round-trips, idempotent
close, claude kind on the wire) plus dispatcher coverage (argv
validation, unknown-id → not-found, text vs bytes_b64, etc). All 37
core tests pass in ~3s under test-core.

Co-Authored-By: Claude <noreply@anthropic.com>

lib/clide.dart

@@ -9,10 +9,14 @@
 library;
 
 export 'src/daemon/dispatcher.dart';
+export 'src/daemon/pane_commands.dart';
 export 'src/ipc/envelope.dart';
 export 'src/ipc/paths.dart';
 export 'src/ipc/schema_v1.dart';
 export 'src/ipc/server.dart';
+export 'src/panes/event_sink.dart';
+export 'src/panes/pane.dart' show Pane, PaneKind;
+export 'src/panes/registry.dart' show PaneRegistry;
 export 'src/pty/errors.dart' show PtyException;
 export 'src/pty/pty.dart';