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/src/ipc/transport.dart

@@ -0,0 +1,73 @@
+/// DaemonTransport (T-331): the seam between the local app and its
+/// backend. The UI's [DaemonClient] talks JSON-lines through a
+/// [DaemonTransport] instead of a hard-coded unix-socket connect, so a
+/// remote transport (SSH-tunnelled agent socket or ssh-exec channel,
+/// T-329/Q-23) can slot in without touching the client's correlation,
+/// reconnect, or event-forwarding logic.
+///
+/// The wire protocol is unchanged either way: one JSON envelope
+/// (IpcRequest/IpcResponse/IpcEvent, see envelope.dart) per line.
+///
+/// Kept Flutter-free — this file runs under plain `dart test`.
+library;
+
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+
+/// How the app reaches its backend. Implementations own endpoint
+/// resolution + connection establishment; the caller owns retry policy
+/// (the client's backoff loop calls [open] again after a failure).
+abstract interface class DaemonTransport {
+  /// Stable, human-readable endpoint description — the unix socket path
+  /// locally, a `ssh://host/path` form remotely. Used for logs, status
+  /// surfaces, and same-endpoint reconnect short-circuits.
+  String get endpoint;
+
+  /// Establish one connection. Throws on failure (caller retries).
+  Future<DaemonConnection> open();
+}
+
+/// One live backend connection carrying JSON-lines both ways.
+abstract interface class DaemonConnection {
+  /// Incoming lines, one JSON envelope each. Done/error signals the
+  /// connection dropped.
+  Stream<String> get lines;
+
+  /// Send one JSON envelope line (the newline is appended here).
+  void writeLine(String line);
+
+  Future<void> close();
+}
+
+/// Today's path: connect to the workspace-derived unix domain socket
+/// (D-70) the in-process IpcServer is bound to.
+class LocalSocketTransport implements DaemonTransport {
+  LocalSocketTransport(this.socketPath);
+
+  final String socketPath;
+
+  @override
+  String get endpoint => socketPath;
+
+  @override
+  Future<DaemonConnection> open() async {
+    final addr = InternetAddress(socketPath, type: InternetAddressType.unix);
+    return _SocketConnection(await Socket.connect(addr, 0));
+  }
+}
+
+class _SocketConnection implements DaemonConnection {
+  _SocketConnection(this._socket);
+
+  final Socket _socket;
+
+  @override
+  Stream<String> get lines => _socket.cast<List<int>>().transform(utf8.decoder).transform(const LineSplitter());
+
+  @override
+  void writeLine(String line) => _socket.writeln(line);
+
+  @override
+  Future<void> close() => _socket.close();
+}