feat(apple): client-selectable compositor in the macOS client

Adopts punktfunk_connect_ex from the compositor-selection batch: a Compositor enum on
PunktfunkConnection (auto/kwin/wlroots/mutter/gamescope, with the host's name aliases
for env parsing), a "Host compositor" picker in Settings (default Automatic — a
concrete choice is honored only if that backend is available host-side), and
PUNKTFUNK_COMPOSITOR / PUNKTFUNK_REMOTE_COMPOSITOR pass-throughs for the autoconnect
dev hook and the remote first-light test. The wire change is backward-compatible
(optional trailing byte), so no behavior changes at the default.

Validated live against the box: host with no compositor env (auto-detect = KWin)
logged "honoring client compositor request compositor=gamescope" and streamed 60/60
decoded frames from the spawned gamescope.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

clients/apple/Sources/PunktfunkClient/SessionModel.swift

@@ -63,6 +63,7 @@ final class SessionModel: ObservableObject {
     var isBusy: Bool { phase != .idle }
 
     func connect(to host: StoredHost, width: UInt32, height: UInt32, hz: UInt32,
+                 compositor: PunktfunkConnection.Compositor = .auto,
                  autoTrust: Bool = false) {
         guard phase == .idle else { return }
         phase = .connecting
@@ -78,7 +79,7 @@ final class SessionModel: ObservableObject {
             let result = Result { try PunktfunkConnection(
                 host: host.address, port: host.port,
                 width: width, height: height, refreshHz: hz,
-                pinSHA256: pin, identity: identity) }
+                pinSHA256: pin, identity: identity, compositor: compositor) }
             await MainActor.run { [weak self] in
                 guard let self else { return }
                 switch result {