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/PunktfunkClient/ContentView.swift

@@ -69,7 +69,9 @@ struct ContentView: View {
             SpeedTestSheet(host: host)
         }
         .sheet(item: $libraryTarget) { host in
-            NavigationStack { LibraryView(store: store, host: host) }
+            NavigationStack {
+                LibraryView(store: store, host: host, onLaunch: { launchTitle(host, $0) })
+            }
         }
         #endif
     }
@@ -80,14 +82,16 @@ struct ContentView: View {
             store: store, model: model, discovery: discovery,
             showAddHost: $showAddHost, pairingTarget: $pairingTarget,
             speedTestTarget: $speedTestTarget, libraryTarget: $libraryTarget,
-            connect: connect, connectDiscovered: connectDiscovered, onPaired: handlePaired)
+            connect: { connect($0) }, connectDiscovered: connectDiscovered,
+            onPaired: handlePaired, onLaunchTitle: launchTitle)
         #else
         HomeView(
             store: store, model: model, discovery: discovery,
             showAddHost: $showAddHost, pairingTarget: $pairingTarget,
             speedTestTarget: $speedTestTarget, libraryTarget: $libraryTarget,
             showSettings: $showSettings,
-            connect: connect, connectDiscovered: connectDiscovered, onPaired: handlePaired)
+            connect: { connect($0) }, connectDiscovered: connectDiscovered,
+            onPaired: handlePaired, onLaunchTitle: launchTitle)
         #endif
     }
 
@@ -171,7 +175,7 @@ struct ContentView: View {
 
     // MARK: - Connect
 
-    private func connect(_ host: StoredHost) {
+    private func connect(_ host: StoredHost, launchID: String? = nil) {
         // The gamepad-type setting resolves NOW (Automatic → match the active physical
         // controller): the host's virtual pad backend is fixed per session.
         model.connect(
@@ -183,7 +187,15 @@ struct ContentView: View {
             gamepad: GamepadManager.shared.resolveType(
                 setting: PunktfunkConnection.GamepadType(
                     rawValue: UInt32(clamping: gamepadType)) ?? .auto),
-            bitrateKbps: UInt32(clamping: bitrateKbps))
+            bitrateKbps: UInt32(clamping: bitrateKbps),
+            launchID: launchID)
+    }
+
+    /// Picked a title in the (experimental) library: dismiss the browser and start a session that
+    /// asks the host to launch it.
+    private func launchTitle(_ host: StoredHost, _ id: String) {
+        libraryTarget = nil
+        connect(host, launchID: id)
     }
 
     /// Tap a discovered host: save it (so the session has a stored identity and the trust pin

clients/apple/Sources/PunktfunkClient/HomeView.swift

@@ -24,6 +24,8 @@ struct HomeView: View {
     let connectDiscovered: (DiscoveredHost) -> Void
     /// Pairing succeeded (tvOS PairSheet route) — pin + connect (ContentView guards staleness).
     let onPaired: (StoredHost, Data) -> Void
+    /// Picked a title in the (experimental) library — start a session that launches it.
+    let onLaunchTitle: (StoredHost, String) -> Void
     /// Experimental game-library browser (gated) — the host-card "Browse Library…" action.
     @AppStorage(DefaultsKey.libraryEnabled) private var libraryEnabled = false
 
@@ -85,7 +87,7 @@ struct HomeView: View {
                 SpeedTestSheet(host: host)
             }
             .navigationDestination(item: $libraryTarget) { host in
-                LibraryView(store: store, host: host)
+                LibraryView(store: store, host: host, onLaunch: { onLaunchTitle(host, $0) })
             }
             #endif
             #if !os(tvOS)

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()

clients/apple/Sources/PunktfunkClient/SessionModel.swift

@@ -87,6 +87,7 @@ final class SessionModel: ObservableObject {
                  compositor: PunktfunkConnection.Compositor = .auto,
                  gamepad: PunktfunkConnection.GamepadType = .auto,
                  bitrateKbps: UInt32 = 0,
+                 launchID: String? = nil,
                  autoTrust: Bool = false) {
         guard phase == .idle else { return }
         phase = .connecting
@@ -103,7 +104,7 @@ final class SessionModel: ObservableObject {
                 host: host.address, port: host.port,
                 width: width, height: height, refreshHz: hz,
                 pinSHA256: pin, identity: identity, compositor: compositor,
-                gamepad: gamepad, bitrateKbps: bitrateKbps) }
+                gamepad: gamepad, bitrateKbps: bitrateKbps, launchID: launchID) }
             await MainActor.run { [weak self] in
                 guard let self else { return }
                 // The user may have abandoned this attempt (window closed, another host

clients/apple/Sources/PunktfunkClient/SettingsView.swift

@@ -412,9 +412,9 @@ struct SettingsView: View {
                 Text("Experimental")
             } footer: {
                 Text("Adds a “Browse Library…” action to each host that lists its games "
-                    + "(Steam + custom) via the host's management API. The host must expose that "
-                    + "API on the LAN with a token (serve --mgmt-bind 0.0.0.0 --mgmt-token …). "
-                    + "Browsing only for now — launching a title comes later.")
+                    + "(Steam + custom) via the host's management API; tap a title to launch it. "
+                    + "The host must expose that API on the LAN with a token "
+                    + "(serve --mgmt-bind 0.0.0.0 --mgmt-token …).")
                     .font(.caption)
                     .foregroundStyle(.secondary)
             }