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/apple/Sources/PunktfunkKit/Connection/PunktfunkConnection.swift

@@ -759,6 +759,17 @@ public final class PunktfunkConnection {
         _ = punktfunk_connection_send_input(h, &ev)
     }
 
+    /// Signal a **deliberate** user-initiated quit before ``close()``: the connection closes with
+    /// `QUIT_CLOSE_CODE` (81) so the host tears the session down immediately instead of holding the
+    /// keep-alive linger for a reconnect. Call only from an explicit "Disconnect" action — NOT from a
+    /// network drop / host-ended / app-background (those keep the linger). Idempotent, safe pre-close.
+    public func disconnectQuit() {
+        abiLock.lock()
+        defer { abiLock.unlock() }
+        guard let h = handle, !closeRequested else { return }
+        punktfunk_connection_disconnect_quit(h)
+    }
+
     /// Close the connection and free the handle. Safe from any thread, idempotent; waits
     /// for in-flight pulls (≤ their timeouts) before tearing down.
     public func close() {