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 (77871d6)

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/linux/src/ui_stream.rs

@@ -817,6 +817,9 @@ fn attach_keyboard(
         // the capture toggle alone can't end a stream, so this is the keyboard's explicit exit.
         if state.contains(chord) && keyval.to_lower() == gdk::Key::d {
             cap.release();
+            // Deliberate user exit → close with QUIT_CLOSE_CODE so the host tears the session down
+            // immediately instead of holding the keep-alive linger for a reconnect.
+            cap.connector.disconnect_quit();
             stop_kb.store(true, Ordering::SeqCst);
             return glib::Propagation::Stop;
         }
@@ -1024,6 +1027,8 @@ fn spawn_disconnect_watch(
     glib::spawn_future_local(async move {
         if disconnect_rx.recv().await.is_ok() {
             cap.release();
+            // Deliberate user exit (the controller escape chord) → QUIT_CLOSE_CODE, host skips linger.
+            cap.connector.disconnect_quit();
             if window.is_fullscreen() {
                 window.unfullscreen();
             }