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/HostStore.swift

@@ -21,8 +21,17 @@ struct StoredHost: Identifiable, Codable, Hashable {
     var pinnedSHA256: Data?
     /// Last time a streaming session actually started (nil until the first one).
     var lastConnected: Date?
+    /// Management-API port for the experimental library browser (distinct from the data-plane
+    /// `port`). Optional (NOT a defaulted non-optional) so older saved hosts — whose JSON lacks
+    /// this key — still decode: synthesized Decodable ignores property defaults but treats a
+    /// missing Optional as nil. Resolve via `effectiveMgmtPort`.
+    var mgmtPort: UInt16?
+    /// Bearer token for the management API (the host's `--mgmt-token`). Required for any
+    /// non-loopback mgmt bind; nil until the user enters it.
+    var mgmtToken: String?
 
     var displayName: String { name.isEmpty ? address : name }
+    var effectiveMgmtPort: UInt16 { mgmtPort ?? punktfunkDefaultMgmtPort }
 }
 
 extension StoredHost {
@@ -87,6 +96,15 @@ final class HostStore: ObservableObject {
         hosts[i].pinnedSHA256 = nil
     }
 
+    /// Persist the management-API endpoint for the (experimental) library browser. An empty
+    /// token is stored as nil (no credential).
+    func setMgmt(_ hostID: UUID, port: UInt16, token: String) {
+        guard let i = hosts.firstIndex(where: { $0.id == hostID }) else { return }
+        hosts[i].mgmtPort = port
+        let trimmed = token.trimmingCharacters(in: .whitespacesAndNewlines)
+        hosts[i].mgmtToken = trimmed.isEmpty ? nil : trimmed
+    }
+
     private func persist() {
         if let data = try? JSONEncoder().encode(hosts) {
             UserDefaults.standard.set(data, forKey: Self.key)