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/native/src/session/connect.rs

@@ -179,6 +179,30 @@ pub extern "system" fn Java_io_unom_punktfunk_kit_NativeBridge_nativeClose(
     })
 }
 
+/// `NativeBridge.nativeDisconnectQuit(handle)` — signal a DELIBERATE user quit before `nativeClose`,
+/// so the session closes with `QUIT_CLOSE_CODE` and the host tears it down immediately instead of
+/// holding the keep-alive linger for a reconnect. Call from an explicit disconnect action only (a
+/// plain drop / app-background keeps the linger). The handle is only BORROWED (not freed). No-op on `0`.
+///
+/// # Safety contract
+/// `handle` must be `0` or a live handle from [`Java_io_unom_punktfunk_kit_NativeBridge_nativeConnect`],
+/// not freed / closed concurrently with this call (Kotlin still owns it and closes it via `nativeClose`).
+#[no_mangle]
+pub extern "system" fn Java_io_unom_punktfunk_kit_NativeBridge_nativeDisconnectQuit(
+    _env: JNIEnv,
+    _this: JObject,
+    handle: jlong,
+) {
+    jni_guard((), || {
+        if handle != 0 {
+            // SAFETY: per the contract, `handle` is a live `Box<SessionHandle>` — we only borrow it
+            // (no drop), so it stays owned by Kotlin for the later `nativeClose`.
+            let sh = unsafe { &*(handle as *const SessionHandle) };
+            sh.client.disconnect_quit();
+        }
+    })
+}
+
 /// `NativeBridge.nativeHostFingerprint(handle): String` — the SHA-256 (64-hex) of the cert the host
 /// presented on this connection. Valid after a successful `nativeConnect`; Kotlin pins it on a TOFU
 /// connect. `""` on a `0` handle.