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 (ede3936).

- 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 ede3936).

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

clients/apple/Sources/PunktfunkClient/LibraryView.swift

@@ -9,6 +9,9 @@ import SwiftUI
 struct LibraryView: View {
     @ObservedObject var store: HostStore
     let host: StoredHost
+    /// Tapping a title starts a session that asks the host to launch it (the library id is passed
+    /// through). `nil` ⇒ browse-only (cards aren't tappable).
+    var onLaunch: ((String) -> Void)? = nil
 
     @State private var games: [GameEntry] = []
     @State private var loading = false
@@ -59,7 +62,12 @@ struct LibraryView: View {
         ScrollView {
             LazyVGrid(columns: columns, spacing: 18) {
                 ForEach(games) { game in
-                    GameCard(game: game)
+                    if let onLaunch {
+                        Button { onLaunch(game.id) } label: { GameCard(game: game) }
+                            .buttonStyle(.plain)
+                    } else {
+                        GameCard(game: game)
+                    }
                 }
             }
             .padding()