route DaemonClient through a DaemonTransport seam (T-331)

The UI's backend client connected straight to the workspace unix
socket, hard-coding the local shape. It now talks JSON-lines through a
DaemonTransport (new lib/src/ipc/transport.dart, Flutter-free), with
LocalSocketTransport reproducing today's connect byte-for-byte — zero
behavior change, proven by the untouched client test suite plus new
seam tests driving the client over an in-memory transport.

This is the slot the SSH-remote backend (T-329/Q-23) plugs into:
request correlation, reconnect/backoff, and event forwarding live
above the seam and won't change when the endpoint is remote.
main.dart's swapIpcServer becomes swapBackend per the same plan.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/helpers/fake_ipc.dart

@@ -4,7 +4,7 @@ import 'package:clide/kernel/kernel.dart';
 /// A DaemonClient that doesn't actually open a socket. Use in tests
 /// that need a connected-state observable but not a real daemon.
 class FakeDaemonClient extends DaemonClient {
-  FakeDaemonClient({required super.log, required super.events}) : super(socketPath: '/dev/null/fake-clide.sock');
+  FakeDaemonClient({required super.log, required super.events}) : super.unixSocket(socketPath: '/dev/null/fake-clide.sock');
 
   bool _fakeConnected = false;
   final Map<String, Future<IpcResponse> Function(Map<String, Object?>)> _stubs = {};