refactor(player): delete the webview video path; mpv selects its own tracks

DR-235 phase 3. Every video renderer is native now: mpv on Linux and
Windows, ExoPlayer on Android, all drawing behind the transparent
webview. The HTML5 <video> path is gone, not bypassed:

- Frontend: hls.js, Html5PlayerAdapter and its compatibility shim, the
  createAdapter factory, streamTransport, hlsRecovery, timeTracking,
  videoFit, the <video>/<track> markup and every element handler in
  VideoPlayer (3277 -> 2144 lines), the experimentalNativeVideo store
  and its Settings toggle, webviewVideoFallback/supportsNativeVideo, and
  the setHtml5VideoState PiP bridge call. NativePlayerAdapter is the one
  video adapter; webview audio gets its own adapter kind.
- Rust: use_html5 dropped from player_seek_video,
  player_switch_audio_track and player_set_stream_quality with the
  Html5* strategies and ReloadStream responses; use_html5_element and
  VideoBackend dropped from PlayerStatus; player_play_item always loads
  the backend (set_current_item removed); Capabilities::webview removed;
  the WebKitGTK GStreamer/VAAPI setup (and its gst-inspect spawn) removed.
- Android: the HTML5 video state in PictureInPictureManager and
  ScreenWakeManager, and the bridge method feeding it.
- CSP: connect-src loses http:/https: and worker-src loses blob: -
  both existed for hls.js; with it gone they were only an exfiltration
  channel and a blob worker for injected script. A test now keeps them
  out.

mpv takes over what the <video> element did (mpv_tracks, UT-275):
subtitles are the WebVTT list the play request carries, queued on
sub-files and selected by position in that list, starting off; audio
tracks are selected by position in the file; sid/aid are reset before
each load. Without this, Linux video had no subtitle selection and a
direct-play audio switch failed since mpv became its renderer.

Verified: Rust 948 passing, and the same 948 cross-compiled for Windows
under wine against the shipped DLL (track tests included); frontend
1111 passing; aarch64 debug APK builds. Lint warnings 158 -> 146, CI
ratchet tightened to match. Not yet seen on Windows hardware.

docs/architecture/02-svelte-frontend.md

@@ -791,36 +791,15 @@ by exactly the inset.
 Unlike `addJavascriptInterface`, the inset push only writes CSS properties, so it
 can safely be re-sent on resume.
 
-## Stream Transport
+## Stream Selection in the Player
 
-**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`.
+**TRACES**: UR-079 | DR-225, DR-227
 
 `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 —
+derived from it. A reload replaces the selection **wholesale**, so transport and
+URL can never drift apart — the transport is the stream's property and comes from
+Rust, never from a substring match on the URL (which is what `.m3u8` sniffing in
+the component once did). 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.
 
@@ -833,21 +812,16 @@ ceiling above the source bitrate *is* the source.
 ## Native Video Store
 
 **Location**: `src/lib/stores/nativeVideo.ts`
-**TRACES**: UR-003, UR-004 | DR-188
+**TRACES**: UR-003, UR-004 | DR-188, DR-235
 
-Two separate concerns live here, deliberately:
+`nativeVideoActive` — whether a native surface is on screen *right now*.
+Setting it toggles `data-native-video` on `<html>`, which is what the CSS in
+`app.css` keys off to clear the app's opaque backgrounds so the picture behind
+the webview shows. The backgrounds must come back the moment the player unmounts.
 
-- `experimentalNativeVideo` — the user-facing opt-in flag, **defaulting to on**.
-  Rust already decides *which backend this platform has* (`useHtml5Element` from
-  `player_play_item`); this flag only *suppresses* that decision. It never turns
-  native on where Rust says HTML5. An explicit stored choice wins in both
-  directions, so someone who opted out is not re-enabled by a default flip —
-  hence the `null` check rather than a bare `=== "true"`.
-- `nativeVideoActive` — whether a native surface is on screen *right now*.
-  Setting it toggles `data-native-video` on `<html>`, which is what the CSS in
-  `app.css` keys off to clear the app's opaque backgrounds. It is deliberately
-  **not** derived from the flag: the backgrounds must come back the moment the
-  player unmounts.
+It used to hold `experimentalNativeVideo`, a stored switch that could force video
+back to the webview `<video>` element. That element is gone (DR-235), and the
+switch with it.
 
 See [05-platform-backends.md](05-platform-backends.md#native-video-compositing-android)
 for what is behind the WebView.