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>

test/helpers/kernel_fixture.dart

@@ -33,7 +33,7 @@ class KernelFixture {
       preloadNamespaces: preloadNamespaces ?? catalogs.keys.toList(),
       defaultLocale: defaultLocale,
       initialLocale: initialLocale,
-      daemonClientFactory: (log, events, _) {
+      daemonClientFactory: (log, events, _, __) {
         fake = FakeDaemonClient(log: log, events: events);
         return fake!;
       },