Enable FFmpeg SPDIF muxer

[cjohnsto-nz]

What changed

• re-enable FFmpeg's spdif muxer after the build disables all muxers
• fail the runtime build unless CONFIG_SPDIF_MUXER is present

Why

mpv audio passthrough uses FFmpeg's SPDIF muxer to packetize AC-3, E-AC-3, TrueHD, DTS, and DTS-HD for IEC 61937 output. The current runtime can select an spdif_* decoder but cannot open the encoded output because the muxer is disabled.

This keeps the existing small dynamic runtime layout; it does not introduce another runtime source or a static libmpv overlay.

Validation

• bash -n build-ffmpeg.sh
• git diff --check upstream/vcpkg...HEAD
• Windows CI passed for x64 and ARM64: https://github.com/cjohnsto-nz/mpv/actions/runs/30734439693
• x64 archive SHA-256: 10a292c000e036d7e0c176c77abedd761dd667498d269470d96815bfbdfcea48
• the x64 artifact was consumed by Soia's normal runtime setup and pnpm bundle:win:release completed successfully
• resulting Soia NSIS installer: 25.23 MiB (the compact dynamic-runtime size class)

[cjohnsto-nz]

@FengZeng Are you able to take a look at this? 🙏

[FengZeng]

@FengZeng Are you able to take a look at this? 🙏

Thanks for working on this! This is a really nice feature, and bringing audio passthrough support back definitely makes sense.

I haven't had much time to focus on the audio side of things yet, so really appreciate you looking into this.