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/windows/src/input.rs

@@ -281,6 +281,9 @@ unsafe extern "system" fn kbd_proc(code: i32, wparam: WPARAM, lparam: LPARAM) ->
                 // the cursor is free while the session winds down and the UI navigates home.
                 if !up && vk == VK_D.0 && st.ctrl && st.alt && st.shift {
                     set_captured(st, false);
+                    // 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.
+                    st.connector.disconnect_quit();
                     st.stop.store(true, Ordering::SeqCst);
                     tracing::info!("disconnect requested (Ctrl+Alt+Shift+D)");
                     return LRESULT(1);