feat(android): play the original file — decode Dolby/DTS audio with FFmpeg

Android ships no AC-3, E-AC-3, DTS or TrueHD decoders; they are licensed
codecs, present only where a vendor paid for them. The ROD2-W09 tablet has a
vendor DTS decoder and no AC-3/E-AC-3 at all. So every film with Dolby audio
was re-encoded by the server, for streaming and for download alike, and a
transcoded download has no Content-Length and ignores Range: ~1 MB/s,
restarting from byte zero on every network blip.

ExoPlayer now carries Jellyfin's media3 FFmpeg audio decoder in extension
mode ON (platform decoders first, FFmpeg for what they lack), and
CodecDetector reports its codecs so the device profile and the download
policy agree with what actually decodes. The download policy judges audio
against the renderer that will play the file (renderer_can_decode_audio)
instead of the webview's list, so Android downloads are always the direct
copy — a 910 MB E-AC-3 5.1 episode downloaded in 94 s and played offline.

The webview video path is removed on Android: it decodes none of these
codecs, so a stored "native video off" would play every original-file
download silent. Rust reports webview_video_fallback (false on Android, true
only beside mpv native video on Linux); Settings offers the switch and the
player honours it only then. Linux keeps the fallback and, with it, the
server transcode for undecodable audio.

The decoder is GPL-3.0; the distributed APK carries its terms and the source
stays MIT (THIRD_PARTY_NOTICES.md). The on-device remux spec this replaces is
folded into 05-platform-backends.md and deleted.

DR-293, UT-259, UT-262.

docs/specs/README.md

@@ -28,8 +28,10 @@ know how something *works*, read
 
 **Next free requirement ids** (always re-check
 [requirements.md](../requirements.md) before allocating): **UR-086**,
-**IR-036**, **JA-038**, **DR-291** (DR-289/290 went to the download stall
-timeout and the transcode progress estimate in v0.12.2). Three specs below suggested ids that have
+**IR-036**, **JA-038**, **DR-295**, **UT-264** (DR-289/290 went to the v0.12.2
+download fixes; DR-291/292 and UT-255/257/258 to the offline-banner and
+server-only-reveal work; DR-293/294 and UT-259-263 to Android's FFmpeg decoding
+and offline-without-network). Three specs below suggested ids that have
 since been taken by other work; each carries a ⚠️ note at the top — this line
 was itself stale by five, two and forty-seven until 2026-09-08, which is why the
 re-check is not optional.
@@ -50,7 +52,6 @@ re-check is not optional.
 |---|---|
 | [desktop-native-video.md](desktop-native-video.md) | mpv draws video on every desktop platform, then the webview `<video>` path and hls.js are deleted. Converts a measured 7% direct-play rate toward Android's 85%. Stacked on backend-owned stream selection. |
 | [backend-owned-stream-selection.md](backend-owned-stream-selection.md) | Rust owns direct-play-vs-transcode, transport and quality; players consume one `StreamSelection`. Partly built — `StreamSelection`, `Transport` and the `.m3u8` sniff removal have landed. |
-| [on-device-audio-remux.md](on-device-audio-remux.md) | Downloads fetch the original (`Static=true`, resumable — measured 14× faster) and a bundled FFmpeg re-encodes only the audio on device, replacing DR-171's server-side transcode. HEVC side effect accepted (Android-first; the webview video path is being retired). Android APK size increase still to be measured. |
 | [build-provenance.md](build-provenance.md) | `build.rs` is still bare. ⚠️ suggested id DR-093 is taken. |
 | [player-facade-enforcement.md](player-facade-enforcement.md) | ~60 `commands.player*` sites still outside the facade; no lint rule. ⚠️ suggested id DR-095 is taken. |
 | [windows-native-audio-backend.md](windows-native-audio-backend.md) | Blocked on the libmpv2 swap. ⚠️ suggested id IR-030 is taken. |
@@ -84,4 +85,5 @@ Where to look for each:
 | Video background audio | [05-platform-backends.md](../architecture/05-platform-backends.md) — Background Audio Handoff |
 | Traceability gate repair | [traceability-ci.md](../traceability-ci.md) |
 | Boundary tripwire hardening | `scripts/check-frontend-boundary.sh` (its header is the spec) |
+| Original-file downloads & Android FFmpeg decoding (was on-device-audio-remux) | [05-platform-backends.md](../architecture/05-platform-backends.md) — Licensed audio codecs; [06-downloads-and-offline.md](../architecture/06-downloads-and-offline.md) — What a Video Download Fetches, Offline Means No Network |
 | Playback docs corrections · req-coverage script removal | Nothing to document — both were corrections that have been applied |