feat(apple/library): experimental game-library browser (flagged off)

Plan step 3 — the Apple client surfaces the host's game library, behind a feature
flag (`DefaultsKey.libraryEnabled`, default OFF). Browsing only; launching a chosen
title is step 4.

- PunktfunkKit `LibraryClient`: Codable GameEntry/Artwork/LaunchSpec mirroring
  crates/punktfunk-host/src/library.rs, and an async fetch of GET /api/v1/library
  with a bearer token. Typed LibraryError guides setup (the common case is "needs a
  --mgmt-token"). `Artwork.posterCandidates` = portrait → header → hero.
- `LibraryView`: cross-platform poster grid (LazyVGrid, AsyncImage that walks the art
  candidates past load failures to a text placeholder), a store badge, and an inline
  Connection form (mgmt port + token) that surfaces when the API is unreachable / 401
  / no token set. Read-only.
- StoredHost gains `mgmtPort`/`mgmtToken` (the mgmt API is a distinct port from the
  data plane and needs a token off-loopback). Both OPTIONAL — synthesized Decodable
  ignores property defaults but treats a missing Optional as nil, so older saved
  hosts decode unchanged (a defaulted non-optional would wipe the list). HostStore.setMgmt.
- Entry point: a flag-gated "Browse Library…" host-card context action → LibraryView
  (sheet on macOS/iOS, pushed on tvOS), mirroring the pair/speed-test plumbing. Plus a
  Settings "Experimental" toggle.

Can't compile Swift on the Linux dev box; CI (apple.yml: swift build + swift test on
the mac mini) verifies the macOS path. Added LibraryClientTests (decode + art order)
for `swift test`. iOS/tvOS-only branches mirror existing patterns. Live-verify on the
Mac pending.

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

clients/apple/Sources/PunktfunkClient/HomeView.swift

@@ -16,6 +16,7 @@ struct HomeView: View {
     @Binding var showAddHost: Bool
     @Binding var pairingTarget: StoredHost?
     @Binding var speedTestTarget: StoredHost?
+    @Binding var libraryTarget: StoredHost?
     #if !os(macOS)
     @Binding var showSettings: Bool
     #endif
@@ -23,6 +24,8 @@ struct HomeView: View {
     let connectDiscovered: (DiscoveredHost) -> Void
     /// Pairing succeeded (tvOS PairSheet route) — pin + connect (ContentView guards staleness).
     let onPaired: (StoredHost, Data) -> Void
+    /// Experimental game-library browser (gated) — the host-card "Browse Library…" action.
+    @AppStorage(DefaultsKey.libraryEnabled) private var libraryEnabled = false
 
     var body: some View {
         NavigationStack {
@@ -81,6 +84,9 @@ struct HomeView: View {
             .navigationDestination(item: $speedTestTarget) { host in
                 SpeedTestSheet(host: host)
             }
+            .navigationDestination(item: $libraryTarget) { host in
+                LibraryView(store: store, host: host)
+            }
             #endif
             #if !os(tvOS)
             .toolbar {
@@ -146,7 +152,8 @@ struct HomeView: View {
     // MARK: - Cards
 
     private func hostCard(_ host: StoredHost) -> some View {
-        HostCardView(
+        let onBrowseLibrary: (() -> Void)? = libraryEnabled ? { libraryTarget = host } : nil
+        return HostCardView(
             host: host,
             isOnline: isOnline(host),
             isConnecting: model.phase == .connecting && model.activeHost?.id == host.id,
@@ -156,7 +163,8 @@ struct HomeView: View {
             onPair: { if !model.isBusy { pairingTarget = host } },
             onSpeedTest: { if !model.isBusy { speedTestTarget = host } },
             onForget: { store.forgetIdentity(host) },
-            onRemove: { store.remove(host) })
+            onRemove: { store.remove(host) },
+            onBrowseLibrary: onBrowseLibrary)
     }
 
     private var discoveredSection: some View {