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/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)
             }