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.

docs/architecture/02-svelte-frontend.md

@@ -802,6 +802,45 @@ by exactly the inset.
 Unlike `addJavascriptInterface`, the inset push only writes CSS properties, so it
 can safely be re-sent on resume.
 
+## Stream Transport
+
+**Location**: `src/lib/player/streamTransport.ts`
+**TRACES**: UR-079 | DR-225 | UT-214
+
+`videoLoaderFor(selection, capabilities)` picks the loader for the webview
+`<video>` element — `hlsjs`, `nativeHls`, or `direct` — from the backend's tagged
+`selection.transport`. `elementSrcFor` is its template companion: the element's
+`src` is emptied only when hls.js is driving it.
+
+The split is the point. **The transport is the stream's property and comes from
+Rust; whether a given loader exists is the browser's, and is the only thing
+decided here.**
+
+> This replaced `currentStreamUrl.includes(".m3u8")`, which appeared twice in
+> `VideoPlayer.svelte` — once in the HLS `$effect` and once inline in the
+> template's `src`. Rust builds that URL and knows what it is; re-deriving it
+> here by substring match was a domain fact reconstructed in the presentation
+> layer, and it fails silently in both directions. The two tests that pin it are
+> the ones that failed against the old implementation: a `progressive` stream
+> whose URL contains `.m3u8` must **not** get an HLS loader, and an `hls` stream
+> whose URL contains no `.m3u8` must.
+>
+> Logic lives in a plain `.ts` module rather than in the component for the usual
+> reason — it is testable there. Same pattern as `episodeStrip.ts`.
+
+`VideoPlayer` holds a `currentSelection`, not a URL string; `currentStreamUrl` is
+derived from it. A reload replaces the selection **wholesale** (the adapter's
+bridge takes a `StreamSelection`, not a URL), so transport and URL can never
+drift apart. The background-audio handoff states the transport it is moving to —
+progressive mp3 out, HLS back — via `selectionAt()`, rather than leaving it to be
+inferred.
+
+The quality picker is filled from `selection.available` (DR-227): rungs the
+backend marked `exceedsSource` are not drawn, because they produce the same bytes
+as `Original`. Nothing is optimistically assigned when the viewer picks a rung —
+what the menu shows comes from the selection the backend hands back, since a
+ceiling above the source bitrate *is* the source.
+
 ## Native Video Store
 
 **Location**: `src/lib/stores/nativeVideo.ts`