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/specs/read-through-media-cache.md

@@ -4,15 +4,20 @@
 (DR-126, DR-127 — a cache entry *is* a `downloads` row with a shorter life, and
 eviction only reclaims the temporary tier), local playback of downloaded media
 (DR-128), and the one-path/one-row invariants that followed (DR-133 … DR-138).
-DR-123 is in progress. Still open: the **player quality selector** and the
-read-through capture itself — DR-121, DR-122, DR-124, DR-125. The separate
-settings-level bitrate cap (DR-162, shipped —
-[01-rust-backend.md](../architecture/01-rust-backend.md#streaming-quality-ladder))
-covers a *settings-level*
-ceiling (DR-162), which serves part of UR-070 but is not the per-playback
-selector specified here.
-**Requirements:** UR-070, UR-071 → DR-121, DR-122, DR-123, DR-124, DR-125; IR-032
-**UX spec:** player quality selector — needs a `ux-flows.md` section before build
+DR-123 is in progress. Still open: the read-through capture itself — DR-122,
+DR-124, DR-125.
+
+**DR-121 has shipped and left this spec.** The player quality selector, the
+per-playback bitrate ceiling, and the backend-owned stream decision it needed
+were built as *backend-owned stream selection* (DR-225 … DR-228) and are
+described in
+[01-rust-backend.md](../architecture/01-rust-backend.md#stream-selection) and
+[03-data-flow.md](../architecture/03-data-flow.md#video-stream-selection-flow).
+The settings-level ceiling (DR-162) is the same section. What remains here is the
+*capture* half only — this spec no longer specifies anything about choosing a
+bitrate.
+
+**Requirements:** UR-070, UR-071 → DR-122, DR-123, DR-124, DR-125; IR-032
 **Related:** the locally-indexed search and downloaded-browse work, both
 shipped — see
 [03-data-flow.md](../architecture/03-data-flow.md) and
@@ -70,24 +75,16 @@ frontend stores the user's *choice*; Rust decides what that choice resolves to.
 
 ## Design
 
-### DR-121 — Bitrate selection in the player
+### DR-121 — moved out (shipped)
 
-The player exposes the qualities Rust reports for the current item. Changing it
-re-negotiates the stream URL at the new quality and resumes at the current
-position. This is a deliberate, user-initiated interruption — a brief rebuffer is
-expected and acceptable, unlike the involuntary swap the earlier design would
-have needed.
+Bitrate selection in the player shipped as DR-225 … DR-228; see
+[01-rust-backend.md](../architecture/01-rust-backend.md#stream-selection).
 
-Constraints that must not be broken:
-
-- On Linux, video playback must keep using the HLS `master.m3u8` URL. CLAUDE.md
-  records that returning `stream.mp4` means transcoded playback never starts.
-  A quality change re-negotiates *within* HLS.
-- The quality→transcode-parameter mapping already exists in
-  `get_video_download_url` ([online.rs:1702-1717](../../src-tauri/src/repository/online.rs#L1702-L1717)).
-  Playback must call into the same mapping. Two copies of that table will drift.
-- Track selection (audio/subtitle) already survives a stream re-negotiation
-  elsewhere in the player; a quality change must preserve it too.
+The one constraint here that the capture work still has to respect: a quality
+change re-negotiates **within HLS**. Returning a progressive `stream.mp4` for a
+transcode means playback never starts, because the server encodes the whole file
+before serving a byte (DR-140). That is why DR-122 below abandons a capture on a
+quality change rather than trying to splice one.
 
 ### DR-122 — The playback path is ephemeral
 
@@ -212,7 +209,6 @@ codec taxonomy in `src/`; the selector's remembered choice is a view preference.
 
 | Piece | Tag |
 |---|---|
-| Quality selector + re-negotiation | `// TRACES: UR-070 \| DR-121` |
 | Ephemeral playback / capture abandonment | `// TRACES: UR-070 \| DR-122` |
 | Independent whole-file download + local video playback fix | `// TRACES: UR-071 \| DR-123, IR-032` |
 | ExoPlayer cache / mpv stream-record / keepability | `// TRACES: UR-071 \| DR-124` |