Link test: UDP echo sweep, soak and TCP each way, driven by the app

The tablet's app sockets are the only ones bound to the USB link, so the
measurement has to run in the app. tool/linktest_server.py serves the
Orin end and logs every result with its own TCP_INFO. Startable without
touching the screen: am start ... --es linktest <label>.

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

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

@@ -1,6 +1,7 @@
 package dev.yukart.display
 
 import android.content.Context
+import android.content.Intent
 import android.net.ConnectivityManager
 import android.net.Network
 import android.net.NetworkCapabilities
@@ -20,12 +21,16 @@ import io.flutter.plugin.common.MethodChannel
  *   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.
+ *
+ * Also a development hook: `am start -n dev.yukart.display/.MainActivity --es linktest <label>`
+ * runs the link test (lib/linktest.dart) without touching the screen.
  */
 class MainActivity : FlutterActivity() {
     private lateinit var channel: MethodChannel
     private lateinit var cm: ConnectivityManager
     private var ethernet: Network? = null
     private var wantBinding = false
+    private var pendingLinkTest: String? = null
 
     private val callback = object : ConnectivityManager.NetworkCallback() {
         override fun onAvailable(network: Network) {
@@ -44,6 +49,12 @@ class MainActivity : FlutterActivity() {
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
         window.addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON)
+        pendingLinkTest = intent?.getStringExtra("linktest")
+    }
+
+    override fun onNewIntent(intent: Intent) {
+        super.onNewIntent(intent)
+        intent.getStringExtra("linktest")?.let { channel.invokeMethod("linktest", it) }
     }
 
     override fun configureFlutterEngine(flutterEngine: FlutterEngine) {
@@ -57,6 +68,10 @@ class MainActivity : FlutterActivity() {
                     apply()
                     result.success(ethernet != null)
                 }
+                "takeLinkTest" -> {
+                    result.success(pendingLinkTest)
+                    pendingLinkTest = null
+                }
                 else -> result.notImplemented()
             }
         }