feat(video): let mpv decode video at all, behind one shared flag

mpv has never decoded a video frame in this app: the backend sets `video: no`
unconditionally, because Linux video has always been the webview's job and
decoding it twice would burn a core for a picture nobody sees. The render path
built in the previous commit therefore had nothing to draw.

With native video on, mpv is configured for video *and* `vo=libmpv` — the render
API only works through that output, and the default would try to open a window
of its own. Set at construction, because mpv resolves its video output when it
initialises and flipping the property later does not re-open one.

The flag lives in `player::native_video`, read by all three things that must
agree: the backend (configured before anything plays), the surface (nothing to
draw otherwise), and `get_player_status` (which tells the frontend whether to
use a `<video>` element — two decoders on one stream would fight over the
audio). A function rather than three `env::var` checks, because a capability
answered in several places is a capability whose answers drift: four separate
bugs this cycle came from exactly that shape.

Also fixes an ordering bug the first run exposed. The surface was attached in
`setup` before the player backend was constructed, and the mpv handle is
registered *during* that construction — so it found nothing every time and
logged "no mpv handle". Attaching after the backend exists is the whole fix.

Confirmed on a real run: mpv accepts `vo=libmpv`, the GL context comes up on
Tauri's vbox, and `mpv_render_context_create` succeeds — which also proves the
libepoxy data-symbol handling is right, since a wrong `get_proc_address` would
have taken SIGSEGV on the first GL call rather than returning cleanly.

No frame has reached the screen yet. The webview is still opaque, so it will
paint over anything drawn beneath it until transparency is set up.

Security: quick-xml 0.38.4 carried RUSTSEC-2026-0194 (quadratic parse on
duplicate attribute names) and RUSTSEC-2026-0195 (unbounded namespace
allocation, memory-exhaustion DoS). `cargo deny` gates CI on advisories, so this
would have failed the next release. Fixed by plist 1.8 -> 1.10, which pulls
quick-xml 0.41. Licences, bans and sources still pass.

UT-216 pins the flag's parsing: absent, empty, `0`, `no` and anything
unrecognised all mean off. A half-set variable that half-enabled the renderer
would configure mpv for video with nothing drawing it — audio over a black
rectangle.

Also removes a wall-clock timer from the waitForRepository late-arrival test,
which failed once under load. The assertion is about ordering, so it now
publishes on a microtask and cannot race.

docs/requirements.md

@@ -739,6 +739,7 @@ Internal architecture, components, and application logic.
 | UT-213 | The direct-play negotiation, one test per branch, against `PlaybackInfo` fixtures whose shapes were all observed on a live server: a supported source direct-plays; a remuxable one direct-streams and reports itself as *not* transcoding; an unsupported codec transcodes; undecodable audio overrides the server's direct-play offer (silent picture is worse than a transcode); a pinned audio track forces a transcode; a ceiling below the source bitrate transcodes even though the codec is fine, and the ladder agrees that rung constrains it; direct play wins over direct stream when both are offered. Plus the ceiling: a per-playback override governs the stream being opened without disturbing the durable default the Settings screen shows, and dropping it returns to that default | DR-225, DR-227 | Done |
 | UT-214 | The loader comes from the transport, never the URL. hls.js is attached for `hls` when available and the element's own loader when not; progressive and local files load directly; the element's `src` is emptied only when hls.js drives it. The two cases that fail against a substring check, and the reason the field exists: a `progressive` stream whose URL contains `.m3u8` is *not* given an HLS loader, and an `hls` stream whose URL contains no `.m3u8` *is*. Both failed against the pre-DR-225 implementation before the fix landed | DR-224 | Done |
 | UT-215 | Waiting for the repository rather than racing it: it resolves immediately when the session is already restored, resolves when the session arrives later (the race the player page lost on mount), still rejects when there genuinely is no session, unsubscribes once settled so a later store change cannot re-settle it, and leaves no armed timer to reject an already-resolved promise | DR-013 | Done |
+| UT-216 | The native-video opt-in is read from one place and only explicit truthy values enable it: absent, empty, `0`, `no`, `false` and anything unrecognised all mean off, because a half-set variable that half-enabled the renderer would configure mpv for video with nothing drawing it — audio over a black rectangle | DR-231 | Done |
 
 ### Integration Tests