feat(apple/library): launch a picked title (step 4 client side)

Tapping a game in the (flagged) library now starts a session that asks the host to
launch it — the picked GameEntry id rides the connect down to the host, which resolves
it against its own library (27e5865).

- PunktfunkConnection.init gains `launchID` and calls the new punktfunk_connect_ex4
  (wrapping it in withOptionalCString; nil = host default).
- Threaded SessionModel.connect(launchID:) → ContentView.connect(_:launchID:) →
  a `launchTitle(host, id)` helper that dismisses the browser and connects.
- LibraryView gains `onLaunch`; cards become buttons that fire it. Wired on every
  platform (ContentView sheet on macOS/iOS, HomeView destination on tvOS) via a new
  `onLaunchTitle` closure on HomeView. Settings footer updated (launch is live now).

Can't compile Swift on the Linux box; CI (apple.yml) verifies. The host side of this
chain is live-validated on the dev box: a client `--launch custom:<id>` made the host
resolve the id and spawn gamescope running the title (see 27e5865).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

clients/apple/Sources/PunktfunkKit/PunktfunkConnection.swift

@@ -242,26 +242,31 @@ public final class PunktfunkConnection {
         compositor: Compositor = .auto,
         gamepad: GamepadType = .auto,
         bitrateKbps: UInt32 = 0,
+        launchID: String? = nil,
         timeoutMs: UInt32 = 10_000
     ) throws {
         if let pin = pinSHA256, pin.count != 32 { throw PunktfunkClientError.invalidPin }
         var observed = [UInt8](repeating: 0, count: 32)
+        // `launchID` (a host library id like "steam:570") asks the host to launch that title in
+        // the session; the host resolves it against its own library — nil = the host's default.
         handle = host.withCString { cs in
             withOptionalCString(identity?.certPEM) { cert in
                 withOptionalCString(identity?.keyPEM) { key in
-                    if let pin = pinSHA256 {
-                        return pin.withUnsafeBytes { p in
-                            punktfunk_connect_ex3(
-                                cs, port, width, height, refreshHz, compositor.rawValue,
-                                gamepad.rawValue, bitrateKbps,
-                                p.bindMemory(to: UInt8.self).baseAddress, &observed,
-                                cert, key, timeoutMs)
+                    withOptionalCString(launchID) { launch in
+                        if let pin = pinSHA256 {
+                            return pin.withUnsafeBytes { p in
+                                punktfunk_connect_ex4(
+                                    cs, port, width, height, refreshHz, compositor.rawValue,
+                                    gamepad.rawValue, bitrateKbps, launch,
+                                    p.bindMemory(to: UInt8.self).baseAddress, &observed,
+                                    cert, key, timeoutMs)
+                            }
                         }
+                        return punktfunk_connect_ex4(
+                            cs, port, width, height, refreshHz, compositor.rawValue,
+                            gamepad.rawValue, bitrateKbps, launch,
+                            nil, &observed, cert, key, timeoutMs)
                     }
-                    return punktfunk_connect_ex3(
-                        cs, port, width, height, refreshHz, compositor.rawValue,
-                        gamepad.rawValue, bitrateKbps,
-                        nil, &observed, cert, key, timeoutMs)
                 }
             }
         }