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/panes/pane.dart

@@ -0,0 +1,60 @@
+/// A single active pane in the daemon.
+///
+/// Owns a [PtySession] plus whatever pane-level metadata the UI +
+/// CLI need. The `kind:` field distinguishes general-purpose terminal
+/// panes from Claude panes (D-041, not yet landed) from whatever
+/// future pane-shaped surface Tier 1+ grows.
+library;
+
+import '../pty/session.dart';
+
+/// Kind of a pane. Keep this enum small and explicit — each kind
+/// typically pairs with a bundled extension that manages its
+/// lifecycle (`builtin.terminal`, `builtin.claude`).
+enum PaneKind {
+  terminal,
+  claude;
+
+  String get wire => name;
+
+  static PaneKind parse(String s) {
+    return PaneKind.values.firstWhere(
+      (v) => v.wire == s,
+      orElse: () => throw ArgumentError.value(s, 'kind', 'unknown pane kind'),
+    );
+  }
+}
+
+/// A live pane. Thin wrapper over [PtySession] — the registry is what
+/// owns the session lifecycle; consumers of this class read state and
+/// call [write] / [resize] via the session.
+class Pane {
+  Pane({
+    required this.id,
+    required this.kind,
+    required this.session,
+    required this.argv,
+    this.cwd,
+    this.title,
+  });
+
+  final String id;
+  final PaneKind kind;
+  final PtySession session;
+  final List<String> argv;
+  final String? cwd;
+  final String? title;
+
+  int get pid => session.pid;
+  bool get isClosed => session.isClosed;
+
+  Map<String, Object?> toJson() => {
+        'id': id,
+        'kind': kind.wire,
+        'pid': pid,
+        'argv': argv,
+        if (cwd != null) 'cwd': cwd,
+        if (title != null) 'title': title,
+        'closed': isClosed,
+      };
+}