Starter display: boundary v0 client, readout, mark button, CI

Connects to svc-gw over one WebSocket, heartbeats, treats timeout_ms of
silence as a dead link, and retries commands under the same id until acked.
Shows link state, signals with stale/null/missing told apart, the raw stream,
a mock banner on the surface, and a Mark button that stamps the recording.
Android: keeps the screen on and binds the process to the USB NCM network
when the gateway is on it. CI mirrors android-stuff's host-mode runner setup.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DDwfd6Ki8UfG4hK6W5YpJz

lib/net.dart

@@ -0,0 +1,34 @@
+// Android only: binding the process to the USB (NCM) Ethernet link to the Orin.
+// See MainActivity.kt for why Android needs telling.
+
+import 'dart:io';
+
+import 'package:flutter/foundation.dart';
+import 'package:flutter/services.dart';
+
+class UsbLink extends ChangeNotifier {
+  static const _ch = MethodChannel('yukart/net');
+
+  bool supported = Platform.isAndroid;
+  bool up = false;
+  bool bound = false;
+
+  UsbLink() {
+    if (!supported) return;
+    _ch.setMethodCallHandler((call) async {
+      if (call.method == 'ethernet') {
+        final a = Map<String, dynamic>.from(call.arguments as Map);
+        up = a['up'] == true;
+        bound = a['bound'] == true;
+        notifyListeners();
+      }
+    });
+  }
+
+  /// Bind only when the target is on the USB link; a workstation over WiFi must stay reachable.
+  Future<void> bindFor(Uri target) async {
+    if (!supported) return;
+    final want = target.host.startsWith('192.168.55.');
+    await _ch.invokeMethod<bool>('bindEthernet', want);
+  }
+}