register live GUI panes into clide pane list (T-219)

Per D-6 parity / D-83, make the kernel tabs the user sees (Claude,
Files, Editor, viewers) visible to the CLI. The PTY-backed PaneRegistry
can't model widget panes, so rather than mirror state (and risk drift),
pane.list snapshots the kernel PanelRegistry + LayoutArrangement at
request time via an injected view-pane source.

New Flutter-free ViewPane value type + snapshotViewPanes kernel bridge;
the daemonClientFactory now passes the PanelRegistry through so the
dispatcher can read it. pane.list merges PTY panes (source absent) with
UI tabs (source: ui, with slot/title/active/visible).

Acceptance (GUI-open: pane list enumerates live panes) needs a running
app to confirm end-to-end; unit-tested at the snapshot + dispatch level.

Closes T-219 (under T-218 / T-208 'Give Claude hands').

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

integration_test/extension_lifecycle_test.dart

@@ -40,7 +40,7 @@ void main() {
         'builtin.ipc-status',
         'builtin.default-layout',
       ],
-      daemonClientFactory: (log, events, _) => FakeDaemonClient(log: log, events: events),
+      daemonClientFactory: (log, events, _, __) => FakeDaemonClient(log: log, events: events),
       autoStartDaemonClient: false,
     );
     services.extensions