feat(pkg): compat.ffmpeg descriptor slimming 17.5k→13.2k (source glob-compression) + include_dirs_after

Optimize the compat.ffmpeg descriptor size (user: 一个文件几千行太夸张):
- sources brace-glob compressed per directory — PR#89's gen_mp.py had
  regressed them to 1-line-per-file (~4300 lines); linux 29 / macosx 26 globs.
- ffmpeg source root ('*','*/libavcodec') → include_dirs_after (-idirafter,
  mcpp#249) so libc++ <version> isn't shadowed by ffmpeg's VERSION on
  case-insensitive macOS — fixes the macOS/Windows C++ module (import ffmpeg.av).

Generator: tools/compat-ffmpeg/gen_multiplatform.py (retains common/delta split
for when mcpp#251 lands — neutral generated_files + per-OS blocks then dedup the
config too, ffmpeg 3-platform → ~9k). Windows ffmpeg lands with that (its link
already works via #247). Parses clean on 0.0.100 xpkg parse --all-os.

Author: Sunrisepeak