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>

lib/kernel/src/facade.dart

@@ -191,7 +191,7 @@ class KernelServices {
         isolateClient ??
         (daemonClientFactory != null
             ? daemonClientFactory(log, events, arrangement, panels)
-            : DaemonClient(
+            : DaemonClient.unixSocket(
                 // Legacy socket-client fallback — kept until T-127
                 // replaces it with the in-process socket loopback.
                 // Today nothing in production hits this branch