async recvFd + backend terminal test (sandbox blocks ptyc exec)

PtySession.spawn() now runs recvFd in a child isolate via
Isolate.spawn so the blocking FFI call doesn't stall the backend
isolate's event loop.

Terminal testmode test spawns a real backend isolate, opens a project,
and sends pane.spawn via IPC — the exact same path the full app uses.
Currently fails: ptyc successfully starts but its fork+execvp is
blocked by the macOS sandbox ("Operation not permitted"). The SBPL
allows /bin/zsh exec from the app process, but ptyc's child process
may not inherit the exec permission, or the PTYC_SOCK_FD is not
inherited by the ptyc child (Dart Process.start fd inheritance on
macOS).

IsolateClient.events getter exposed for test access.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/kernel/src/ipc/isolate_client.dart

@@ -26,6 +26,9 @@ class IsolateClient extends DaemonClient {
 
   final SendPort _backendPort;
   final DaemonBus _events;
+
+  /// The event bus that receives events from the backend.
+  DaemonBus get events => _events;
   final Map<String, Completer<IpcResponse>> _pending = {};
   int _nextId = 0;