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 (b71dc94)

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/PunktfunkClient/Session/SessionModel.swift

@@ -276,7 +276,10 @@ final class SessionModel: ObservableObject {
         disconnect()
     }
 
-    func disconnect() {
+    /// Tear the session down. `deliberate` (the default) means a user-initiated quit — signal
+    /// `disconnectQuit()` so the host skips the keep-alive linger; `sessionEnded()` (a host-ended /
+    /// dropped session) passes `false` to leave the linger intact.
+    func disconnect(deliberate: Bool = true) {
         statsTimer?.invalidate()
         statsTimer = nil
         let audio = self.audio
@@ -294,6 +297,8 @@ final class SessionModel: ObservableObject {
             Task.detached {
                 audio?.stop()
                 feedback?.stop()
+                // Deliberate user quit → tell the host to skip the keep-alive linger (must precede close).
+                if deliberate { conn.disconnectQuit() }
                 conn.close()
             }
         } else {
@@ -321,7 +326,7 @@ final class SessionModel: ObservableObject {
     func sessionEnded() {
         guard connection != nil else { return }
         let name = activeHost?.displayName ?? "host"
-        disconnect()
+        disconnect(deliberate: false) // host/network ended it — keep the linger for a reconnect
         errorMessage = "Session ended by \(name)."
     }