feat(player): native video on Linux, and one contract for every player (v0.11.0)

mpv now decodes video on Linux, drawn into a framebuffer we own and blitted
into the default vbox's draw handler. Tauri's widget tree is untouched, so an
upgrade that assumes its own layout cannot invalidate this. Direct play means
the original file, hardware decoding, and no server transcode at all — where
previously every desktop video was re-encoded to h264 for the browser engine,
whatever the file actually was. Off by default: JELLYTAU_NATIVE_VIDEO=1.

That settles finding 2 of playback-backend-unification.md — "native video
cannot be composited with a Tauri webview" — by demonstration rather than
argument, on X11 and Wayland both.

Turning it on exposed nine defects, none of them mpv's. Each was the same
mistake in a different place: a capability written down as a compile-time fact
about the platform, or a state asserted instead of confirmed.

  DR-238/246  a seek routed by the stream's container rather than by what the
              engine could do with it - correct only while one player handled
              those streams, silent the moment another did
  DR-239      a property handled but never observed, so the play/pause button
              waited for an event that could not arrive
  DR-240      fullscreen expanding the document while the window stayed put
  DR-241      a seek issued before the engine had a file, failed, and discarded
              - which is why resume began at zero
  DR-247      a Linux-only gate outliving the caller that made it Linux-only,
              breaking the Android build outright
  DR-250      a stop aimed at whichever renderer bookkeeping believed was in
              charge, missing the one actually making sound
  DR-251      a duration of zero believed, leaving the seek bar no scale
  DR-252      a junk float converted to a Duration, panicking the backend the
              instant a length-less stream appeared

So the MediaPlayer contract (DR-242 … DR-247): `open` carries a start position,
so no caller sequences load-then-seek and none can race an engine's load;
`seek` states a destination and leaves in-place-versus-re-open to the engine;
`snapshot` is one coherent read; and `Phase::Opening` names the window where
intent used to be lost. One conformance suite runs against every engine —
FakePlayer and mpv under cargo test, ExoPlayer instrumented on a device — so an
engine is either correct or visibly failing.

Two of the nine were introduced during this work and caught on hardware, not by
any suite: an over-broad capability that grouped ExoPlayer with mpv, and the
Duration panic. The suites test engines that behave. That is recorded in
docs/native-player-verification.md, which asks for the exact action sequences
that found them.

Verified: all automated gates, conformance (mpv 9/9, legacy 8/9 by design,
ExoPlayer 7/7 on device), and manual desktop and Android passes on real
hardware.

Known open and deliberately shipped: resume reads local progress and never the
server's; the background-audio handoff still declares a state swap it does not
confirm (the symptom is now impossible, the race is not); and `bun run
android:dev` builds an APK carrying the release application id, whose failure
message advises an uninstall that would destroy app data. Fix that last one
before anyone else builds for Android.

Squashed from worktree-linux-native-video, which keeps the per-defect history.

src/lib/player/localSource.ts

@@ -1,41 +1,3 @@
-/**
- * Choosing between a downloaded file and a server stream for video playback.
- *
- * Audio has preferred local files since the queue is built (the Rust queue
- * resolves `MediaSource::Local`), but video asks the repository for a stream URL
- * and never consults `downloads` — so a downloaded film was streamed anyway,
- * spending bandwidth that had already been spent and failing outright offline.
- *
- * Pure so it can be unit-tested: the component only supplies the two inputs and
- * the asset-URL converter.
- *
- * TRACES: UR-071 | DR-123 | UT-118
- */
-
-export interface VideoSourceInputs {
-  /** Absolute on-disk path of a completed download, or null to stream. */
-  localPath: string | null;
-  /** Stream URL the repository resolved (already transcoded if it had to be). */
-  remoteUrl: string;
-  /** Whether the *remote* stream is a transcode. */
-  remoteNeedsTranscoding: boolean;
-  /** Usually Tauri's `convertFileSrc`; injected so this module stays pure. */
-  toAssetUrl: (path: string) => string;
-}
-
-export interface VideoSourceDecision {
-  /** What to hand the `<video>` element. */
-  url: string;
-  /**
-   * Local files are never transcodes, so this is always false for them. It
-   * matters because the transcoded path re-requests a whole new stream URL on
-   * every seek; a local file seeks natively and must not go down that route.
-   */
-  needsTranscoding: boolean;
-  /** True when playing from disk — for logging and the offline badge. */
-  isLocal: boolean;
-}
-
 /** Absolute on POSIX (`/…`), Windows (`C:\…`, `C:/…`) or a UNC share (`\\…`). */
 function isAbsolute(path: string): boolean {
   return path.startsWith("/") || path.startsWith("\\") || /^[A-Za-z]:[\\/]/.test(path);
@@ -57,15 +19,3 @@ function isAbsolute(path: string): boolean {
 export function downloadedFilePath(storageRoot: string, filePath: string): string {
   return isAbsolute(filePath) ? filePath : `${storageRoot}/${filePath}`;
 }
-
-export function resolveVideoSource(inputs: VideoSourceInputs): VideoSourceDecision {
-  const { localPath, remoteUrl, remoteNeedsTranscoding, toAssetUrl } = inputs;
-
-  // Treat blank/whitespace paths as absent — a malformed `downloads` row must
-  // not produce an asset URL pointing at nothing.
-  if (localPath && localPath.trim() !== "") {
-    return { url: toAssetUrl(localPath), needsTranscoding: false, isLocal: true };
-  }
-
-  return { url: remoteUrl, needsTranscoding: remoteNeedsTranscoding, isLocal: false };
-}