feat(clients): signal explicit exit (QUIT_CLOSE_CODE) on deliberate disconnect

The core's deliberate-quit close (NativeClient::disconnect_quit → QUIT_CLOSE_CODE,
host skips the keep-alive linger) was implemented but never called by any client.
Wire it to each client's explicit user-disconnect action — NOT to a network drop /
host-ended / app-background (those keep the linger for a reconnect):

- core: new C-ABI punktfunk_connection_disconnect_quit(c) for the ABI clients
- Linux (direct-core): Ctrl+Alt+Shift+D + the controller escape chord
- Windows (direct-core): Ctrl+Alt+Shift+D
- Apple (C-ABI): PunktfunkConnection.disconnectQuit() + a `deliberate` flag on
  SessionModel.disconnect() (sessionEnded passes false → keeps the linger)
- Android (JNI): new nativeDisconnectQuit export, called from the back gesture +
  the Select+Start+L1+R1 chord (not the host-gone watchdog)
- probe already did this via --quit (b71dc94)

Verified: core + Linux client + Android (cargo-ndk + gradle) build clean;
Windows/Apple compile-checked by CI.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

clients/android/app/src/main/kotlin/io/unom/punktfunk/StreamScreen.kt

@@ -150,7 +150,9 @@ fun StreamScreen(handle: Long, micEnabled: Boolean, onDisconnect: () -> Unit) {
         activity?.requestedOrientation = ActivityInfo.SCREEN_ORIENTATION_SENSOR_LANDSCAPE
         activity?.streamHandle = handle // route hardware keys to this session
         activity?.axisMapper = Gamepad.AxisMapper(handle) // route joystick axes
-        activity?.requestStreamExit = onDisconnect // Select+Start+L1+R1 chord leaves the stream
+        // Select+Start+L1+R1 chord leaves the stream — a deliberate quit (signal it so the host skips
+        // the keep-alive linger), unlike a host-ended / backgrounded drop.
+        activity?.requestStreamExit = { NativeBridge.nativeDisconnectQuit(handle); onDisconnect() }
         activity?.setConsoleHighRefreshRate(false) // let the decoder's setFrameRate pick the panel rate
         // Host→client feedback (rumble + DualSense lightbar/LEDs); poll threads stopped before close.
         val feedback = GamepadFeedback(handle).also { it.start() }
@@ -179,7 +181,8 @@ fun StreamScreen(handle: Long, micEnabled: Boolean, onDisconnect: () -> Unit) {
         }
     }
 
-    BackHandler { onDisconnect() }
+    // Back gesture = a deliberate exit → signal the quit so the host tears down now (no linger).
+    BackHandler { NativeBridge.nativeDisconnectQuit(handle); onDisconnect() }
 
     Box(modifier = Modifier.fillMaxSize()) {
         AndroidView(