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>

include/punktfunk_core.h

@@ -1501,6 +1501,18 @@ PunktfunkStatus punktfunk_connection_probe_result(const PunktfunkConnection *c,
                                                   PunktfunkProbeResult *out);
 #endif
 
+#if defined(PUNKTFUNK_FEATURE_QUIC)
+// Signal a **deliberate quit** (a user "stop", not a network drop) before closing: the connection
+// closes with [`QUIT_CLOSE_CODE`] instead of code 0, so the host tears the session down immediately
+// (skips the keep-alive linger) rather than holding it for a reconnect. Call this right before
+// [`punktfunk_connection_close`] on a user-initiated disconnect; a plain close (network drop,
+// backgrounding) leaves the linger intact. NULL is a no-op.
+//
+// # Safety
+// `c` was returned by [`punktfunk_connect`] and remains valid (closed via `punktfunk_connection_close`).
+void punktfunk_connection_disconnect_quit(PunktfunkConnection *c);
+#endif
+
 #if defined(PUNKTFUNK_FEATURE_QUIC)
 // Close the connection and free the handle (joins the internal threads). NULL is a no-op.
 //