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/Tests/PunktfunkKitTests/RemoteFirstLightTests.swift

@@ -53,11 +53,16 @@ final class RemoteFirstLightTests: XCTestCase {
             throw XCTSkip("set PUNKTFUNK_REMOTE_HOST (and start m3-host --source virtual there)")
         }
         let port = env["PUNKTFUNK_REMOTE_PORT"].flatMap(UInt16.init) ?? 9777
+        // PUNKTFUNK_REMOTE_COMPOSITOR=kwin|gamescope|… asks the host for a specific
+        // backend (verify in its log: "punktfunk/1 virtual display compositor=…").
+        let compositor = env["PUNKTFUNK_REMOTE_COMPOSITOR"]
+            .flatMap(PunktfunkConnection.Compositor.init(name:)) ?? .auto
         let width: UInt32 = 1280
         let height: UInt32 = 720
 
         let conn = try PunktfunkConnection(
-            host: host, port: port, width: width, height: height, refreshHz: 60)
+            host: host, port: port, width: width, height: height, refreshHz: 60,
+            compositor: compositor)
         defer { conn.close() }
         XCTAssertEqual(conn.width, width)
         XCTAssertEqual(conn.height, height)