Enable ffmpeg-bins2 manifest: full-feature 8.1.2 for macOS/Linux (Windows/musl fall back to legacy)

[zackees]

Enables the new full-feature FFmpeg 8.1.2 build (built via zackees/forge, published to zackees/ffmpeg-bins2) for the platforms that build cleanly, while keeping every existing install working.

What this does

• Points DEFAULT_MANIFEST_URL at the published ffmpeg-bins2 catalog.
• macOS x64/arm64 and glibc-Linux x64/arm64 now resolve to the new GPL+nonfree, full-codec 8.1.2 build (x264, x265, fdk-aac, vpx, aom, dav1d, svt-av1, mp3lame, opus, vorbis, openh264, openjpeg, webp, …), downloaded from CDN-backed GitHub Release assets and sha256-verified.
• Windows and musl are intentionally absent from the catalog (their forge builds are blocked on a systemic MSVC pkg-config issue and forge's C-only musl toolchain, tracked in Migrate to full-feature FFmpeg build (forge) + ffmpeg-bins2 + manifest.json catalog, preserving legacy installs #20). Those clients — and any manifest fetch/parse failure — fall back to the legacy ffmpeg_bins URLs, so nothing regresses.

Verified

• Resolution tested against the live manifest: macOS→darwin zips, glibc-Linux→linux zips; musl/Windows correctly resolve to None (→ legacy fallback).
• All 4 release assets serve HTTP 200.
• flake8/mypy clean, pytest tests/test_manifest.py green.
• This PR's CI is the real end-to-end test: the macOS/Linux workflows download the new binaries via the manifest and run the small-encode test (test_encode.py) on them.

Part of #20.

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