feat(player): render Android video natively behind a transparent webview (DR-150, DR-151, DR-152)

Rust already reported `use_html5_element: false` on Android, but two frontend
overrides threw that answer away, so ExoPlayer's video path had never actually
run. Both are lifted behind an `experimentalNativeVideo` opt-in (default off).

The flag is a suppressor, never a promoter: off forces HTML5 even where Rust
says native, so an in-progress spike cannot ship as the default, but it can
never select native where Rust reported HTML5 — Linux cannot composite behind
WebKitGTK, and promoting there would be a black screen.

Two blockers the spec did not anticipate, both in code assumed to be merely
unreachable rather than broken:

- `JellyTauPlayer.setActivity()` had zero callers, so `currentActivity` was
  always null and `autoAttachSurface()` bailed. The SurfaceView was created and
  wired to ExoPlayer but never added to the view hierarchy — video would have
  decoded to a surface that was never on screen, whatever the webview did.
  This also revives PiP on the video path, which gated on the same flag.
- `createAdapter()` was not the real gate; it is never called in production.
  The actual override was in VideoPlayer.svelte, which forced HTML5 and stopped
  the native backend `player_play_item` had just started. Both sites now route
  through `createAdapter()`.

Compositing needs two independent opaque layers cleared, not one. Clearing only
the page leaves the WebView widget opaque — audio over a black picture, exactly
the symptom the old INTERIM comment described. `videoSurface.ts` toggles both:
the widget background and window drawable from Kotlin, the page backgrounds via
a `data-native-video` attribute keyed by app.css. Transparency lives in
`tauri.android.conf.json` so Linux keeps an opaque window, and is scoped to the
playback session so the launcher never shows through the rest of the app.

Phase 3's rect plumbing turned out to be unnecessary: video is fullscreen on the
player route, and `fitSurfaceToScreen()` already letterboxes and re-fits on
rotation. The mini-player transition remains unverified on device.

Also removes the `navigator.userAgent` sniffing in webviewAudio.ts, which was a
second copy of the Rust cfg gate free to drift from it. `player_get_capabilities`
now reports `usesWebviewAudio` and `supportsNativeVideo` from those same gates.

Tests: adapter selection covers the full matrix, including the regression guard
that the flag off beats Rust. Written first and confirmed failing (2 of 7) before
the fix.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/lib/player/adapters/adapterSelection.test.ts

@@ -0,0 +1,95 @@
+/**
+ * Adapter-selection regression guards.
+ *
+ * TRACES: UR-003, UR-004 | DR-150 | UT-149
+ *
+ * The selection rule has two inputs and one hard safety property:
+ *
+ *   - Rust says which backend the platform has (`backendKind`).
+ *   - The user opts in with `experimentalNativeVideo`.
+ *   - **The flag off must force HTML5 even when Rust says native.** That is the
+ *     regression guard: a broken spike must not be able to ship as the default.
+ *
+ * These are pure functions, so the whole matrix is testable without a device.
+ */
+
+import { describe, expect, it } from "vitest";
+import { createAdapter } from "./index";
+import { Html5PlayerAdapter } from "./html5Adapter";
+import { NativePlayerAdapter } from "./nativeAdapter";
+import type { AdapterHost } from "./types";
+
+const host: AdapterHost = {
+  reportState: () => {},
+  reportPosition: () => {},
+  reportEnded: () => {},
+} as unknown as AdapterHost;
+
+const bridge = {
+  getElement: () => null,
+} as any;
+
+describe("createAdapter", () => {
+  it("returns the native adapter when Rust says native and the flag is on", () => {
+    const adapter = createAdapter({
+      backendKind: "native",
+      host,
+      bridge,
+      experimentalNativeVideo: true,
+    });
+    expect(adapter).toBeInstanceOf(NativePlayerAdapter);
+    expect(adapter.kind).toBe("native");
+  });
+
+  // The regression guard: the flag is a suppressor, so off must beat Rust.
+  it("forces HTML5 when the flag is off even though Rust says native", () => {
+    const adapter = createAdapter({
+      backendKind: "native",
+      host,
+      bridge,
+      experimentalNativeVideo: false,
+    });
+    expect(adapter).toBeInstanceOf(Html5PlayerAdapter);
+    expect(adapter.kind).toBe("html5");
+  });
+
+  it("returns the HTML5 adapter when Rust says html5 and the flag is off", () => {
+    const adapter = createAdapter({
+      backendKind: "html5",
+      host,
+      bridge,
+      experimentalNativeVideo: false,
+    });
+    expect(adapter).toBeInstanceOf(Html5PlayerAdapter);
+  });
+
+  // The flag must never *promote* a platform Rust said has no native backend
+  // (e.g. Linux, where WebKitGTK cannot composite a surface behind the webview).
+  it("stays on HTML5 when Rust says html5 even with the flag on", () => {
+    const adapter = createAdapter({
+      backendKind: "html5",
+      host,
+      bridge,
+      experimentalNativeVideo: true,
+    });
+    expect(adapter).toBeInstanceOf(Html5PlayerAdapter);
+  });
+
+  it("defaults to HTML5 when the flag is omitted entirely", () => {
+    const adapter = createAdapter({ backendKind: "native", host, bridge });
+    expect(adapter).toBeInstanceOf(Html5PlayerAdapter);
+  });
+
+  it("requires a bridge for the HTML5 adapter", () => {
+    expect(() =>
+      createAdapter({ backendKind: "html5", host, experimentalNativeVideo: false })
+    ).toThrow(/bridge/i);
+  });
+
+  // The native adapter owns no DOM element, so it must not demand a bridge.
+  it("does not require a bridge for the native adapter", () => {
+    expect(() =>
+      createAdapter({ backendKind: "native", host, experimentalNativeVideo: true })
+    ).not.toThrow();
+  });
+});

src/lib/player/adapters/index.ts

@@ -2,13 +2,22 @@
  * 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`.
+ * Rust decides *which backend this platform has* (`useHtml5Element` from
+ * `player_play_item`); this factory consumes that decision rather than
+ * re-deriving it.
  *
- * TRACES: UR-003 | DR-004
+ * The `experimentalNativeVideo` flag is a **suppressor, never a promoter**: it
+ * can force the HTML5 path when Rust says native (so an in-progress spike cannot
+ * ship as a regression), but it can never select native on a platform whose Rust
+ * backend reported HTML5 — Linux has no way to composite a surface behind a
+ * WebKitGTK webview, so promoting there would produce a black screen.
+ *
+ * The previous unconditional HTML5 override cited tauri#10152 as an upstream
+ * blocker. That was stale: #10152 is a dormant *feature request*, the capability
+ * shipped in tauri 27d01834, and the black-screen bug (tauri#8381, #9408) was a
+ * broken `setBackgroundColor` JNI signature fixed in wry 0.39.4 — we ship 0.53.x.
+ *
+ * TRACES: UR-003, UR-004 | DR-004, DR-150 | UT-149
  */
 
 import { Html5PlayerAdapter, type Html5ElementBridge } from "./html5Adapter";
@@ -29,27 +38,36 @@ export interface CreateAdapterArgs {
   host: AdapterHost;
   /** Required for the HTML5 adapter; ignored by the native adapter. */
   bridge?: Html5ElementBridge;
+  /**
+   * User opt-in for the native video path. Defaults to **off**, so omitting it
+   * yields today's behaviour (HTML5 everywhere) rather than silently enabling
+   * the spike.
+   */
+  experimentalNativeVideo?: boolean;
 }
 
 /**
  * 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.
+ * Native is chosen only when Rust reports a native backend AND the user has
+ * opted in. Every other combination is HTML5.
  */
-export function createAdapter({ backendKind, host, bridge }: CreateAdapterArgs): PlayerAdapter {
-  // INTERIM OVERRIDE: force HTML5 rendering even when the backend reports native.
-  const effectiveKind: BackendKind = "html5";
+export function createAdapter({
+  backendKind,
+  host,
+  bridge,
+  experimentalNativeVideo = false,
+}: CreateAdapterArgs): PlayerAdapter {
+  const effectiveKind: BackendKind =
+    backendKind === "native" && experimentalNativeVideo ? "native" : "html5";
 
-  if (effectiveKind === "html5") {
-    if (!bridge) {
-      throw new Error("createAdapter: Html5ElementBridge is required for the HTML5 adapter");
-    }
-    return new Html5PlayerAdapter(host, bridge);
+  if (effectiveKind === "native") {
+    // The native surface is owned by the backend — no DOM element, no bridge.
+    return new NativePlayerAdapter(host);
   }
 
-  // Reached only once the interim override is lifted (native Android unblocked).
-  void backendKind;
-  return new NativePlayerAdapter(host);
+  if (!bridge) {
+    throw new Error("createAdapter: Html5ElementBridge is required for the HTML5 adapter");
+  }
+  return new Html5PlayerAdapter(host, bridge);
 }