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/src/ipc/server.dart

@@ -8,8 +8,7 @@ typedef RequestDispatcher = Future<IpcResponse> Function(IpcRequest request);
 
 /// Unix-socket JSON-lines server. Each connection is an independent
 /// bidirectional line-framed stream: client writes requests, daemon
-/// writes responses (and events, later). Tier 0 handles request→response
-/// only; event broadcasting lands with the first feature that emits.
+/// writes responses + events on the same socket.
 class DaemonServer {
   DaemonServer({
     required this.socketPath,
@@ -22,6 +21,23 @@ class DaemonServer {
   ServerSocket? _server;
   final Set<Socket> _clients = {};
 
+  /// Broadcast [event] to every currently-connected client.
+  ///
+  /// Future tuning: per-client subsystem/id filter (`tail --filter
+  /// pane:p_7`). For Tier 1 every client sees everything. Sockets
+  /// that error on write are silently dropped; the client's read side
+  /// will notice the close.
+  void broadcast(IpcEvent event) {
+    final line = event.encode();
+    for (final c in List<Socket>.from(_clients)) {
+      try {
+        c.writeln(line);
+      } catch (_) {
+        _clients.remove(c);
+      }
+    }
+  }
+
   Future<void> start() async {
     final addr = InternetAddress(socketPath, type: InternetAddressType.unix);
     try {