feat(windows): mpv plays audio on Windows, with libmpv shipped in the installer

libmpv on Windows (DR-237):
- The builder image carries zhongfly's LGPL libmpv-2.dll, pinned by
  asset name and sha256, plus an MSVC mpv.lib generated from the DLL's
  own mpv_* exports (the archive ships only a MinGW .dll.a). LGPL, not
  the GPL builds: no x264/x265, mpv -Dgpl=false; FFmpeg is version3, so
  LGPL-3.0. THIRD_PARTY_NOTICES.md records it.
- build-windows-cross.sh stages both files into src-tauri/windows-libs/;
  build.rs links mpv.lib from there and tauri.windows.conf.json bundles
  the DLL beside jellytau.exe from there, with the licence texts under
  licenses/. One directory, so the DLL shipped is the one linked.
- Workflows move to builder image 2026.09.1.

Windows audio:
- MpvBackend replaces WebviewAudioBackend on Windows (ao=wasapi), so
  volume, EQ, normalization and gapless work there as on Linux. Local
  files are passed to mpv as native paths, not file:// URLs.

Fixes found on the way:
- player_play_item decided "does the backend render video" with
  cfg!(not(linux)), true on Windows, while get_player_status sent
  Windows video to the <video> element. With mpv as the backend that
  would decode every film's soundtrack twice. All three callers now
  ask video_renders_natively() (UT-273).
- confine_queued_path rebuilt paths with PathBuf::push, so on Windows
  a queued `downloads/x` was stored as `downloads\x`, no longer the
  spelling the app built. It now keeps the caller's separator (UT-205,
  which only ever ran on Linux, failed under Windows).

Verified: jellytau.exe imports libmpv-2.dll; the full unit suite
cross-compiled for Windows passes under wine against the shipped DLL
(943/943, including the mpv injection and TLS tests); the NSIS
installer contains the DLL and licence texts. Not yet run on real
Windows hardware.

docs/build/build-windows.md

@@ -7,17 +7,31 @@ job / SMTC lockscreen), but it runs and plays media.
 
 ## How playback works on Windows
 
-- **Video** — renders through the webview HTML5 `<video>` element (hls.js) on
-  *every* platform; on Windows that is WebView2 (Chromium/Edge), which plays HLS +
-  h264 fine. No Windows-specific code.
-- **Audio-only (music)** — the native audio backends are libmpv (Linux) and
-  ExoPlayer (Android); neither exists on Windows. Instead
-  `create_player_backend()` in [../src-tauri/src/lib.rs](../../src-tauri/src/lib.rs)
-  uses `WebviewAudioBackend` on non-Linux/non-Android targets: it hands the stream
-  URL to a webview `<audio>` element (see
-  [../src/lib/services/webviewAudio.ts](../../src/lib/services/webviewAudio.ts)),
-  which reports state back through the same `player_report_*` round-trip the video
-  path uses. Pure Rust + Tauri events.
+- **Video** — renders through the webview HTML5 `<video>` element (hls.js); on
+  Windows that is WebView2 (Chromium/Edge), which plays HLS + h264 fine. mpv
+  takes this over in DR-237's video phase; Linux already made that move.
+- **Audio** — **libmpv**, the same `MpvBackend` Linux uses, with `ao=wasapi`.
+  Volume, EQ, normalization and gapless all go through mpv's filter graph as on
+  Linux. `libmpv-2.dll` ships beside `jellytau.exe` in the installer.
+
+### libmpv on Windows
+
+- **Which build:** zhongfly/mpv-winbuild's *LGPL* dev asset, pinned by name and
+  sha256 in [Dockerfile.builder](../../Dockerfile.builder), which unpacks it to
+  `/opt/libmpv-win64`. Licence terms: [THIRD_PARTY_NOTICES.md](../../THIRD_PARTY_NOTICES.md).
+  The same release also carries a GPL asset (x264/x265) — do not switch to it
+  casually, it moves the installer onto GPL-3.0 terms.
+- **Import library:** the archive only ships a MinGW `libmpv.dll.a`. The image
+  generates an MSVC `mpv.lib` from the DLL's own `mpv_*` exports
+  (`llvm-readobj --coff-exports` → `.def` → `llvm-lib /def:`), so it cannot name
+  a symbol the DLL lacks.
+- **One directory:** `scripts/build-windows-cross.sh` copies both files into
+  `src-tauri/windows-libs/` (gitignored) before building. `build.rs` links
+  `mpv.lib` from there and `tauri.windows.conf.json` bundles the DLL from there,
+  so the DLL shipped is the one linked against. Outside the image, point
+  `LIBMPV_WIN_DIR` at a directory holding `libmpv-2.dll` and `mpv.lib`.
+- **Bumping it:** change the three `LIBMPV_WIN_*` values together, rebuild and
+  push the image, bump the image tag the workflows pin.
 
 ## Cross-compiling from Linux (MSVC + cargo-xwin)
 
@@ -35,7 +49,7 @@ Tauri CLI bundle the **NSIS installer from a Linux host**.
 
 The builder image ([../Dockerfile.builder](../../Dockerfile.builder)) bakes in the
 whole toolchain: the `x86_64-pc-windows-msvc` rust target, `cargo-xwin`, `lld`,
-`llvm`, and `nsis`.
+`llvm`, `nsis`, and the pinned Windows libmpv.
 
 ```bash
 bun run docker:build:windows            # NSIS installer + .exe -> ./dist
@@ -71,8 +85,6 @@ Outputs:
 
 ## Outstanding for a first-class Windows release
 
-1. Gapless/crossfade + SMTC (lockscreen) — currently no-ops in the webview audio
-   path.
-2. Downloaded (`Local` source) file playback needs `convertFileSrc` on the
-   frontend; streaming works today.
-3. Code signing + a Windows packaging CI job.
+1. SMTC (lockscreen / media keys) — not wired on Windows.
+2. Code signing — the installer is unsigned.
+3. Video through mpv (DR-237) — needs a WebView2-side surface.