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.

src-tauri/Cargo.toml

@@ -108,9 +108,10 @@ tiny_http = { version = "0.12.0", default-features = false }
 tauri-plugin-updater = "2"
 tauri-plugin-process = "2"
 
-# Linux-specific dependencies
-[target.'cfg(target_os = "linux")'.dependencies]
-hostname = "0.4"
+# mpv platforms: Linux (system libmpv) and Windows (libmpv-2.dll shipped in the
+# installer, import library generated in the builder image — see build.rs and
+# scripts/build-windows-cross.sh). TRACES: UR-004 | DR-237
+[target.'cfg(any(target_os = "linux", target_os = "windows"))'.dependencies]
 libc = "0.2"
 # The crates.io release of libmpv predates the MPV versions we support, so this
 # tracks the upstream git repo.
@@ -137,6 +138,9 @@ libmpv = { git = "https://github.com/ParadoxSpiral/libmpv-rs.git", rev = "3e6c38
 # TRACES: UR-080 | DR-230, IR-033
 libmpv-sys = { git = "https://github.com/ParadoxSpiral/libmpv-rs.git", rev = "3e6c389b716f52a595cc5e8e3fa1f96cb76b3de7" }
 
+# Linux-specific dependencies
+[target.'cfg(target_os = "linux")'.dependencies]
+hostname = "0.4"
 # Same major as the one Tauri/wry already resolve, so `gtk_window()` and
 # `default_vbox()` hand back types this crate can name rather than a second,
 # incompatible GTK.