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/Tests/PunktfunkKitTests/LibraryClientTests.swift

@@ -0,0 +1,63 @@
+// Unit tests for the game-library models — decoding the management API's GET /api/v1/library
+// payload and the poster-art fallback order. (The network fetch itself isn't unit-tested; it's
+// exercised live against a host.)
+
+import XCTest
+@testable import PunktfunkKit
+
+final class LibraryClientTests: XCTestCase {
+    func testDecodesLibraryPayload() throws {
+        // A Steam entry (full art + launch) and a custom entry (sparse art, no launch) — the two
+        // shapes the host's `GameEntry` serializes (note the host omits null fields).
+        let json = """
+        [
+          {
+            "id": "steam:570",
+            "store": "steam",
+            "title": "Dota 2",
+            "art": {
+              "portrait": "https://cdn.cloudflare.steamstatic.com/steam/apps/570/library_600x900.jpg",
+              "hero": "https://cdn.cloudflare.steamstatic.com/steam/apps/570/library_hero.jpg",
+              "logo": "https://cdn.cloudflare.steamstatic.com/steam/apps/570/logo.png",
+              "header": "https://cdn.cloudflare.steamstatic.com/steam/apps/570/header.jpg"
+            },
+            "launch": { "kind": "steam_appid", "value": "570" }
+          },
+          {
+            "id": "custom:abc123",
+            "store": "custom",
+            "title": "Dolphin",
+            "art": { "header": "https://example.com/dolphin.jpg" }
+          }
+        ]
+        """.data(using: .utf8)!
+
+        let games = try JSONDecoder().decode([GameEntry].self, from: json)
+        XCTAssertEqual(games.count, 2)
+
+        let steam = games[0]
+        XCTAssertEqual(steam.id, "steam:570")
+        XCTAssertFalse(steam.isCustom)
+        XCTAssertEqual(steam.launch?.kind, "steam_appid")
+        XCTAssertEqual(steam.launch?.value, "570")
+
+        let custom = games[1]
+        XCTAssertTrue(custom.isCustom)
+        XCTAssertNil(custom.launch)
+        XCTAssertNil(custom.art.portrait)
+    }
+
+    func testPosterCandidatesPreferPortraitThenHeader() {
+        let full = Artwork(
+            portrait: "https://x/p.jpg", hero: "https://x/hero.jpg",
+            logo: "https://x/logo.png", header: "https://x/h.jpg")
+        XCTAssertEqual(full.posterCandidates.map(\.absoluteString),
+                       ["https://x/p.jpg", "https://x/h.jpg", "https://x/hero.jpg"])
+
+        // No portrait → header leads; absent fields are skipped, not nil-padded.
+        let sparse = Artwork(portrait: nil, hero: nil, logo: nil, header: "https://x/h.jpg")
+        XCTAssertEqual(sparse.posterCandidates.map(\.absoluteString), ["https://x/h.jpg"])
+
+        XCTAssertTrue(Artwork().posterCandidates.isEmpty)
+    }
+}