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/android/kit/src/main/kotlin/io/unom/punktfunk/kit/NativeBridge.kt

@@ -82,6 +82,14 @@ object NativeBridge {
         name: String,
     ): String
 
+    /**
+     * Signal a **deliberate** user disconnect on [handle] before [nativeClose]: the session closes
+     * with `QUIT_CLOSE_CODE` so the host tears it down immediately instead of holding the keep-alive
+     * linger for a reconnect. Call from an explicit disconnect gesture only — NOT from a
+     * host-ended/network-drop end or an app-background (those keep the linger). No-op on `0`.
+     */
+    external fun nativeDisconnectQuit(handle: Long)
+
     /** Tear down a session handle returned by [nativeConnect]. No-op on `0`. */
     external fun nativeClose(handle: Long)