feat(pkg): compat.ffmpeg multi-platform (linux-x86_64 + macosx-arm64)

Promote the proven multi-platform FFmpeg 8.1.2 source-build descriptor to
production: per-OS frozen config snapshots under mcpp.<os> (linux NASM SIMD /
2282 sources, macosx NEON / 2091 sources), consumers build from source with
zero configure/make. Byte-identical to the PR#87 spike descriptor that passed
both linux + macos legs green.

- pkgs/c/compat.ffmpeg.lua: xpm.{linux,macosx} + per-OS mcpp blocks
- tests/examples/ffmpeg: widen dep gate to cfg(macos) (asserts full surface —
  same 358 demuxers + h264/hevc/av1/aac decoders on both OSes)
- windows deferred to mcpp#247 (driver-style link rspfile)

Author: Sunrisepeak