fix(player): recover a failed stream on Linux instead of stopping (DR-130)

A recoverable player error meant "playback is over": the frontend's error
handler stopped the player unconditionally, so a wifi blip killed the
track. Android already decides in its JNI callback, but MpvBackend is
constructed before PlayerController exists, so its event thread has no
controller to ask.

So MPV reports the failure and the frontend echoes it into the new
player_recover_stream command — the same shape as PlaybackEnded ->
player_on_playback_ended, keeping the decision in Rust. The command
re-opens the stream where it stopped, with the existing attempt budget
and backoff, and returns whether it handled it; only a false answer
falls through to the old stop path.

Android now reports the errors it has already declined as
*unrecoverable*, so the echo never asks the same question twice.

TRACES: UR-004, UR-040 | DR-130 | UT-117

docs/requirements.md

@@ -298,6 +298,7 @@ Internal architecture, components, and application logic.
 | DR-127 | A cache entry *is* a download with a shorter life: same `downloads` row and same file handling, distinguished by `download_source = 'auto'` plus an expiry, so there is one storage model rather than a cache and a download library that can disagree. Temporary rows are reclaimed on whichever comes first — the life limit elapsing, or eviction under space pressure (DR-126). Permanent (`'user'`) rows have no expiry. A temporary row can be promoted to permanent by the user choosing to keep it, which only clears the expiry and flips the source; the bytes never move | Storage | UR-071 | Done |
 | DR-128 | Audio-only playback of *downloaded* media reads the local file rather than fetching an audio-only stream. No transcode is involved or wanted: the Linux backend already runs MPV with `video: no`, so handing it the downloaded video file decodes the audio track and ignores the video, and ExoPlayer disables its video renderer equivalently. Transcoding to a separate audio artifact would cost CPU and battery, need an encoder the project does not ship, and produce a second file to keep in step — for no gain over simply not decoding the video | Playback | UR-071 | Done |
 | DR-129 | A stream that stops delivering is recovered, not treated as terminal. Two failure shapes, because the streams differ. (a) *Phantom end* — the background audio-only handoff uses a progressive mp3 transcode over plain HTTP, chunked and therefore length-less, so a dropped connection reaches the player as end-of-input and ExoPlayer reports `STATE_ENDED` indistinguishably from the real end. The item's runtime is the only thing that can tell them apart: an end reported more than a tolerance short of it (comparing the *absolute* position — handoff base plus the player's relative position) is a truncation. Left unhandled, playback parked in `STATE_ENDED` and the next play intent from the lockscreen, notification or a Bluetooth reconnect seeks an ended player to position 0 — the user-visible "the episode randomly restarted". (b) *Recoverable error* — music (`/Audio/{id}/stream?Static=true`) and video (`/Videos/{id}/master.m3u8`) declare their length, so the player detects the truncation itself and raises an error; the frontend's handler stopped playback outright, turning a hiccup into silence. Both resume the current item **in place** (never via `play_item`, which would replace the queue with a single item and lose the album), the error path after a per-attempt backoff. Seekable streams are re-prepared at the URL they already have and seeked; the length-less transcode, which cannot be seeked, has `StartTimeTicks` rewritten into its existing URL so the user's audio-track selection survives and recovery needs no network round-trip. Only `Remote` sources qualify — a local file cannot fail from the network. A shared budget of consecutive attempts at the same position, refilled whenever playback progresses, stops an unreachable server from looping | Playback | UR-040, UR-004 | Done |
+| DR-130 | A backend's position and duration must survive the end of the file they describe. MPV exposes `time-pos`/`duration` as properties of the *loaded* file, so at EOF it unloads and both stop resolving — the accessors reported `0.0`/unknown at exactly the moment end-of-file handling asks where playback reached, and any position-versus-runtime check would have read every natural end as a truncation. The poll thread records the last reading and the accessors fall back to it. Linux resilience is layered on the same principle that the stream, not the player, is what failed: MPV is configured with ffmpeg reconnection (`stream-lavf-o`, `network-timeout`) so ordinary blips never surface, and `EndFile(ERROR)` — previously a bare log, which left playback halted while the UI still showed "playing" — is emitted as a *recoverable* error. Because MpvBackend is constructed before `PlayerController` exists, it cannot decide in-process like the Android JNI callback: the frontend echoes the error into `player_recover_stream`, which keeps the decision in Rust (the same shape as `PlaybackEnded` → `player_on_playback_ended`). Android reports errors it has already declined as *unrecoverable*, so the echo never asks twice | Playback | UR-004, UR-040 | Done |
 | DR-093 | Traceability coverage gate derives its requirement denominators from `requirements.md` at run time rather than hardcoded literals: `countDefinedRequirements` counts an ID only where it leads a markdown table row (ignoring the "Traces To" column and prose) and deduplicates IDs listed both in the definition tables and in the §3 traceability matrix; `computeCoverage` reports the *intersection* of traced and defined IDs so an ID traced in code but absent from `requirements.md` is surfaced as `orphaned` instead of inflating the ratio past 100%. UT/IT test identifiers are excluded as a separate taxonomy. CI and `bun run traces:coverage` share this computation and fail on both a sub-threshold and an impossible >100% result | Tooling | - | Done |
 
 ---
@@ -347,7 +348,7 @@ Internal architecture, components, and application logic.
 | UR-037 | IR-010 | DR-042 |
 | UR-038 | IR-010 | DR-043 |
 | UR-039 | - | DR-045, DR-046 |
-| UR-040 | IR-025 | DR-051, DR-052, DR-129 |
+| UR-040 | IR-025 | DR-051, DR-052, DR-129, DR-130 |
 | UR-041 | IR-026 | DR-053 |
 | UR-042 | IR-009, IR-014 | DR-054 |
 | UR-043 | IR-027 | DR-055 |
@@ -504,6 +505,7 @@ Internal architecture, components, and application logic.
 | UT-120 | Expiry reclaim takes only expired temporary entries: derived from `completed_at`+TTL, honouring an `expires_at` override, never a user download, and disabled by a zero TTL | DR-127 | Done |
 | UT-108 | LRU eviction reclaims only `'auto'` downloads and never a user's own, even when the user's is the oldest | DR-126 | Done |
 | UT-117 | A background audio-only stream cut short resumes where it died instead of ending the episode; a real end still advances; the absolute position is compared against the runtime; retries at a stuck position give up. A recoverable error resumes music and video too, with growing backoff, leaving the rest of the queue intact and the seekable stream's URL untouched; local and DirectUrl sources are excluded | DR-129 | Done |
+| UT-121 | An EOF reads as the last observed timestamp, not zero: live readings win while the file is loaded, a not-yet-established duration is not recorded as a real zero, a seek updates the position before the next poll, and loading a new file clears the previous one's | DR-130 | Done |
 
 ### Integration Tests