feat(pkg): compat.ffmpeg 3-platform + descriptor slim 17.5k→9k (fix: mcpp_generated/ key prefix)

The earlier avconfig.h-not-found failures were a generator bug of mine, NOT
mcpp: gen_multiplatform.py emitted generated_files keys WITHOUT the
mcpp_generated/ prefix, so they materialized off the -I include path. Fixed →
neutral dedup + common/delta split now materialize correctly under
mcpp_generated/ (locally verified: avconfig.h + config.base.h land there).

So the full data/logic-separation slimming lands: one descriptor, 3 platforms
(linux/macosx/windows), 9,027 lines (was 17,543 for 2). Windows ffmpeg link
works via mcpp#247. include_dirs_after (#249) fixes the macOS C++ module.
mcpp#251 was my misdiagnosis — closing it.

Author: Sunrisepeak