Introduce PlayerAdapter contract; decision logic shared in Rust backend

Establish a decoupled player boundary so UI and backend interact with video
through one contract, with the HTML5 (Linux/interim-Android) and native
(ExoPlayer) providers as interchangeable primitive-executor adapters.

- PlayerAdapter interface + AdapterHost callback bag (adapters/types.ts): the
  adapter owns only decision-free element PRIMITIVES (seekElement, reloadSource,
  play/pause, setVolume, selectSubtitle); it never branches on strategy.
- Seek/audio-track DECISIONS stay in Rust (player_seek_video / _switch_audio_track
  return a strategy); the facade dispatches the chosen primitive to the active
  adapter. Both providers share the one decision path — logic lives once, in Rust.
- Facade holds the active adapter; a new ControlCommand PlayerStatusEvent lets
  backend control (lockscreen/remote/sleep) drive the webview <video> element.
- Html5PlayerAdapter resolves the LIVE element via the bridge (fixes play/pause
  silently no-opping when the element was re-bound).
- Do not emit a "stopped" player state on natural end-of-video: it flipped the
  player/mode to idle mid-handoff and suppressed next-episode auto-advance under
  a sleep timer. Jellyfin progress reporting is preserved; the backend's
  on_video_playback_ended owns the transition.
- VideoPlayer net -300 lines (strategy/HLS-reload logic relocated to the adapter).
- Adds 20 adapter unit tests; existing suites stay green (vitest 457, cargo 416).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/lib/player/adapters/index.ts

@@ -0,0 +1,55 @@
+/**
+ * Player adapter factory + public exports.
+ *
+ * `createAdapter` selects the concrete PlayerAdapter for the current platform.
+ * It is the single place that encodes the INTERIM Android override: the Rust
+ * backend may report a native ExoPlayer backend, but native Android video
+ * rendering is blocked upstream (tauri#10152 — transparent webview / SurfaceView
+ * compositing), so we render Android video through the HTML5 adapter for now.
+ * When that upstream limitation is resolved, flip this to honor `backendKind`.
+ *
+ * TRACES: UR-003 | DR-004
+ */
+
+import { Html5PlayerAdapter, type Html5ElementBridge } from "./html5Adapter";
+import { NativePlayerAdapter } from "./nativeAdapter";
+import type { AdapterHost, PlayerAdapter } from "./types";
+
+export type { PlayerAdapter, AdapterHost, PlayerLoadOptions, SubtitleTrackInput } from "./types";
+export type { Html5ElementBridge } from "./html5Adapter";
+export { Html5PlayerAdapter } from "./html5Adapter";
+export { NativePlayerAdapter } from "./nativeAdapter";
+
+/** What the Rust `player_play_item` response says it chose. */
+export type BackendKind = "html5" | "native";
+
+export interface CreateAdapterArgs {
+  /** Backend kind reported by `player_play_item` (`useHtml5Element`). */
+  backendKind: BackendKind;
+  host: AdapterHost;
+  /** Required for the HTML5 adapter; ignored by the native adapter. */
+  bridge?: Html5ElementBridge;
+}
+
+/**
+ * Build the adapter for this platform/stream.
+ *
+ * INTERIM: always returns the HTML5 adapter, because the native surface is not
+ * visible through the webview on current Tauri (see module docs). The bridge is
+ * therefore required.
+ */
+export function createAdapter({ backendKind, host, bridge }: CreateAdapterArgs): PlayerAdapter {
+  // INTERIM OVERRIDE: force HTML5 rendering even when the backend reports native.
+  const effectiveKind: BackendKind = "html5";
+
+  if (effectiveKind === "html5") {
+    if (!bridge) {
+      throw new Error("createAdapter: Html5ElementBridge is required for the HTML5 adapter");
+    }
+    return new Html5PlayerAdapter(host, bridge);
+  }
+
+  // Reached only once the interim override is lifted (native Android unblocked).
+  void backendKind;
+  return new NativePlayerAdapter(host);
+}