fix(player): three defects from review, and one duplicate removed

Verified each against the code before acting; four of the five findings held,
one did not.

DR-253 — a deferred seek outlived its file. `seek` holds a position while MPV
has nothing loaded and `FileLoaded` applies it (DR-241), but neither `load` nor
`stop` discarded it. Scrub near the end of a transcoded item — which re-opens
the stream — then skip to the next item before the reload completes, and the
old position lands on the new item. It starts wherever the previous one was
scrubbed to, silently. Both lifecycle points clear it now.

DR-254 — a per-playback quality ceiling outlived its playback. The override is
process-wide and describes one playback: dropping to 720p for a struggling
episode says nothing about the next. Every advance the frontend drives clears
it through player_play_item, but the background audio-only advance loads the
next episode in Rust and skipped all three clearing sites — so every later
episode stayed capped, with nothing in the UI explaining why.

DR-255 — `playable_url` was a byte-identical copy of `playback_url`, added for
the cross-platform open path. The original is `#[cfg(target_os = "android")]`,
so it does not exist in a Linux build and nothing warned. Two matches over
MediaSource meant a new variant could be handled in one and forgotten in the
other. The gate is gone and the copy with it.

The fifth finding — that the comment on `video_audio_codecs` describes a
renderer switch the code no longer has — does not hold. `get_player_status`
hard-codes Android to Native, but `experimentalNativeVideo` is still live in
VideoPlayer.svelte as a suppressor that can force HTML5 even when Rust says
native. The switch exists, so the narrow codec list is still doing its job.

Both correctness fixes are red-then-green. The tests are wiring assertions in
the style of UT-218: what matters is the call site, and reaching these at
runtime needs a live MPV handle or a repository, a server and a player. That
technique now appears three times and is worth watching — it pins call sites,
not behaviour.

The review's sharpest point is one it raised as redundancy: MpvPlayer already
handles DR-253 correctly, resetting deferred state on every open, and the old
path had to be patched separately. That is the drift two parallel engines
produce, and the argument for finishing DR-248/249 rather than leaving
LegacyPlayer in place indefinitely.

795 Rust tests, 1088 frontend, every CI check green locally.

docs/requirements.md

@@ -446,6 +446,9 @@ Internal architecture, components, and application logic.
 | DR-250 | Stopping means nothing is playing, from any renderer — not "whatever we believe owns playback has been asked to stop". A background-audio handoff swaps which renderer that is, and the swap is bookkeeping that can be mid-flight: `exit_background_audio` marks the webview element the player again the moment it is called, while the element has not reloaded. The teardown's stop was gated on flags describing what the component started, so after a handoff it described a player that was no longer making sound and the stop was skipped — the audio stream kept running and the mini player adopted it, which is why a movie reappeared as an audio track. The stop is now unconditional (it is idempotent) and clears the handoff base and flag, so a later position read cannot be interpreted against a handoff that no longer exists | Player | UR-040, UR-005 | Done |
 | DR-251 | A duration of zero is treated as "the engine does not know yet", and falls back to the runtime the item already carries. ExoPlayer reports `C.TIME_UNSET` until it resolves one and `JellyTauPlayer.getDuration()` maps that to `0.0`, so the engine answered `Some(0.0)` rather than `None` — which satisfied every "unknown duration" fallback and left the seek bar with no scale. It presented as scrubbing being broken rather than as a duration that never arrived, and the catalog had the runtime the whole time | Player | UR-005, UR-040 | Done |
 | DR-252 | Seconds reported by an engine are converted to a `Duration` only when finite and positive. `Duration::from_secs_f64` panics on a negative or non-finite value and no engine promises otherwise: ExoPlayer reports `C.TIME_UNSET` (`Long::MIN_VALUE`, about -9.2e15) for a stream whose length it does not know, which is every background-audio handoff — `/Audio/{id}/universal` is a chunked, length-less transcode. Held as a float that junk was harmless; converted to a `Duration` by the `MediaPlayer` adapter it became a panic that killed the backend mid-handoff and left a black screen with no controls. One guard on the contract, used by every engine crossing into it | Player | UR-005 | Done |
+| DR-253 | A deferred seek is discarded when the file it was issued against stops being the one loading. `seek` holds a position while MPV has nothing loaded and the `FileLoaded` handler applies it (DR-241), but neither `load` nor `stop` cleared it — so scrubbing near the end of a transcoded item, which re-opens the stream, and then skipping to the next item before the reload completed applied the old position to the new item. It started wherever the previous one had been scrubbed to, silently | Player | UR-040, UR-005 | Done |
+| DR-254 | Advancing to the next episode drops a per-playback quality override. The override is process-wide and describes one playback: a viewer who drops to 720p for a struggling episode has said nothing about the next. Every advance the frontend drives clears it via `player_play_item`; the background audio-only advance loads the next episode in Rust and skipped all three clearing sites, so every later episode stayed capped with nothing in the UI saying why | Repository | UR-074 | Done |
+| DR-255 | One helper answers "what URL should an engine open". `playback_url` was gated to Android because only ExoPlayer needed it, and that gate is why a byte-identical copy was later added for the cross-platform open path — the original is invisible in a Linux build, so nothing warned. Two matches over `MediaSource` meant a new variant could be handled in one and forgotten in the other | Player | UR-081 | Done |
 | DR-198 | The webview runs under a real Content-Security-Policy, and the asset protocol is scoped to the one directory it still serves. `csp` was `null`, which disables CSP entirely: any script that reached the web layer — through a future `{@html}`, a dependency, or a devtools paste — would have inherited the whole IPC surface, and with it the user's session. `script-src 'self'` (Tauri injects a nonce for SvelteKit's inline bootstrap script at build time, so no `'unsafe-inline'` is needed) plus `object-src`/`frame-src 'none'` and `base-uri 'self'` is the part that is genuinely restrictive. `img-src`/`media-src`/`connect-src` cannot be: the Jellyfin origin is typed in by the user at run time and is commonly plain `http` on a LAN, so they allow `http:`/`https:` — a wide grant for *data*, but one that still bars `file:`, `filesystem:` and scripting schemes, and leaves `script-src` untouched. `style-src` keeps `'unsafe-inline'` because Svelte compiles `style="…"` attributes (including `app.html`'s `display: contents` wrapper) into markup; this is safe only while no `<style>` element survives into `index.html`, since a nonce there would make Tauri's injection outrank — and therefore void — `'unsafe-inline'`. `worker-src blob:` and `media-src blob:` are hls.js: it demuxes in a worker built from a blob and attaches MSE through `URL.createObjectURL`. `asset:` and `http://asset.localhost` are the same protocol under the two naming schemes `convertFileSrc` emits (custom scheme on Linux/macOS, `http` host on Windows/Android); `ipc:`/`http://ipc.localhost` is the invoke transport, which would otherwise be blocked by `connect-src`. A run-time CSP naming the server origin exactly was rejected: Tauri computes the header from immutable config when it serves the HTML, so it would mean rebuilding config and reloading the webview on every server change, for a policy the user can already point anywhere. The asset-protocol scope narrows from `$APPDATA/**` to `$APPDATA/thumbnails/**` — since DR-137 moved downloaded media to the loopback server, `imageCache` is the only `convertFileSrc` caller left, so the database and the encrypted-token fallback file no longer sit inside the grant | Security | UR-012, UR-071 | Done |
 
 ---
@@ -762,6 +765,8 @@ Internal architecture, components, and application logic.
 | UT-222 | The values that killed the backend are rejected rather than converted: `C.TIME_UNSET` as seconds, negatives, zero, NaN and both infinities all yield no duration, while a real runtime survives | DR-252 | Done |
 | UT-223 | The adapter survives an engine that answers badly. A `HostileBackend` reports `C.TIME_UNSET` as seconds, NaN, both infinities, a negative and a zero; reading a snapshot yields no duration and a zero position rather than panicking, and a well-behaved engine still round-trips. The conformance suite could not have caught this — it only ever drives engines that report sane numbers, which is why it stayed green while a real one took the backend down | DR-252 | Done |
 | UT-224 | Stopping clears an active background-audio handoff, both the flag and the base offset, so a later position read cannot be interpreted against a handoff that no longer exists. Previously verified only by listening to a device | DR-250 | Done |
+| UT-225 | Both `load` and `stop` discard a deferred seek, so a position held for a file that is no longer loading cannot be applied to whatever loads next | DR-253 | Done |
+| UT-226 | The background episode advance clears the per-playback quality override, so a ceiling chosen for one episode does not cap every episode after it | DR-254 | Done |
 
 ### Integration Tests