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

android/app/src/main/AndroidManifest.xml

@@ -1,6 +1,10 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android">
+    <uses-permission android:name="android.permission.INTERNET"/>
+    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
+    <!-- requestNetwork for the USB (NCM) Ethernet link to the Orin -->
+    <uses-permission android:name="android.permission.CHANGE_NETWORK_STATE"/>
     <application
-        android:label="display"
+        android:label="Yukart"
         android:name="${applicationName}"
         android:icon="@mipmap/ic_launcher">
         <activity

android/app/src/main/kotlin/dev/yukart/display/MainActivity.kt

@@ -1,5 +1,83 @@
 package dev.yukart.display
 
+import android.content.Context
+import android.net.ConnectivityManager
+import android.net.Network
+import android.net.NetworkCapabilities
+import android.net.NetworkRequest
+import android.os.Bundle
+import android.view.WindowManager
 import io.flutter.embedding.android.FlutterActivity
+import io.flutter.embedding.engine.FlutterEngine
+import io.flutter.plugin.common.MethodChannel
 
-class MainActivity : FlutterActivity()
+/**
+ * Two things Dart cannot do on its own:
+ *
+ * - Keep the screen on. An idle tablet suspends about a minute after screen-off and
+ *   then answers nothing, while the Orin still sees the USB link up.
+ * - Route over the USB link. It has no internet, so Android never makes it the
+ *   default network while WiFi or cellular is up, and ordinary sockets go elsewhere.
+ *   When asked, the whole process is bound to the Ethernet (NCM) network while it
+ *   exists, and unbound when it goes.
+ */
+class MainActivity : FlutterActivity() {
+    private lateinit var channel: MethodChannel
+    private lateinit var cm: ConnectivityManager
+    private var ethernet: Network? = null
+    private var wantBinding = false
+
+    private val callback = object : ConnectivityManager.NetworkCallback() {
+        override fun onAvailable(network: Network) {
+            ethernet = network
+            apply()
+        }
+
+        override fun onLost(network: Network) {
+            if (ethernet == network) {
+                ethernet = null
+                apply()
+            }
+        }
+    }
+
+    override fun onCreate(savedInstanceState: Bundle?) {
+        super.onCreate(savedInstanceState)
+        window.addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON)
+    }
+
+    override fun configureFlutterEngine(flutterEngine: FlutterEngine) {
+        super.configureFlutterEngine(flutterEngine)
+        cm = getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
+        channel = MethodChannel(flutterEngine.dartExecutor.binaryMessenger, "yukart/net")
+        channel.setMethodCallHandler { call, result ->
+            when (call.method) {
+                "bindEthernet" -> {
+                    wantBinding = call.arguments as? Boolean ?: false
+                    apply()
+                    result.success(ethernet != null)
+                }
+                else -> result.notImplemented()
+            }
+        }
+        // NET_CAPABILITY_INTERNET is in the default request; the USB link lacks it.
+        val request = NetworkRequest.Builder()
+            .addTransportType(NetworkCapabilities.TRANSPORT_ETHERNET)
+            .removeCapability(NetworkCapabilities.NET_CAPABILITY_INTERNET)
+            .build()
+        cm.requestNetwork(request, callback)
+    }
+
+    private fun apply() {
+        val target = if (wantBinding) ethernet else null
+        cm.bindProcessToNetwork(target)
+        val up = ethernet != null
+        runOnUiThread { channel.invokeMethod("ethernet", mapOf("up" to up, "bound" to (target != null))) }
+    }
+
+    override fun onDestroy() {
+        runCatching { cm.unregisterNetworkCallback(callback) }
+        cm.bindProcessToNetwork(null)
+        super.onDestroy()
+    }
+}