feat(pkg): compat.opencv full-platform full-function — windows videoio + macOS dnn (mcpp 0.0.101)

[Sunrisepeak]

Makes compat.opencv full-platform full-function on mcpp 0.0.101: windows gains the FFmpeg videoio backend, macOS-arm64 gains the dnn module, and the dnn feature is restructured per-OS (mcpp#253). Consumers just do opencv = { features = ["dnn"] } and it works on linux/macOS (windows videoio available now; windows dnn is a follow-up).

What changed

Windows videoio (the deep HAVE_FFMPEG handoff — solved).
OpenCV's videoio gates cap_ffmpeg.cpp on the CMake target ocv.3rdparty.ffmpeg (created by ocv_add_external_target in detect_ffmpeg.cmake), not on the HAVE_FFMPEG variable — so forcing the variable never helped. Fix: the sanctioned OPENCV_FFMPEG_USE_FIND_PACKAGE=FFMPEG + a FindFFMPEG.cmake shim, which sets HAVE_FFMPEG=TRUE before the target is created. CI confirms TARGET ocv.3rdparty.ffmpeg CREATED → cap_ffmpeg: YES (2 TUs) under clang-cl.

macOS dnn (per-OS NEON).
The dnn feature's payload is now split (mcpp#253 per-OS features): neutral features.dnn carries the cross-platform common payload (dnn/protobuf/mlas C++ + mlas_hgemm_stub), and each OS's SIMD delta rides mcpp.<os>.features.dnn — x86 (mlas/lib/x86_64/*.S + avx/avx2/avx512 kernels) on linux/windows, arm (mlas/lib/aarch64/*.S + neon/neon_fp16 kernels) on macOS. Snapshot built core+imgproc+…+dnn on macos-15 (NEON): 70 .neon.cpp kernels.

Dead-glob warning cleared (mcpp 0.0.101 per-feature flags).
dnn-group flag-globs (mlas/protobuf/mlasgemm + per-ISA) moved out of the feature-off base flags into features.dnn.flags, so they no longer matched no source file when dnn is off.

Tooling

• gen_descriptor.py: route dnn flag-globs → features.dnn.flags; fix empty-baseline (arm) cxxflags = { , "-w" } lua syntax bug.
• merge_opencv.lua: per-OS dnn common/delta split + flag relocation (format-robust across pre/post-warnfix inputs).
• Snapshot workflows: windows/macOS full profile + dnn.

Validation

• mcpp 0.0.101 xpkg parse clean on all 3 OSes.
• opencv-dnn + opencv-module-dnn test members widened to macOS — this PR's workspace (macos) job builds the full dnn closure from source and runs the blobFromImage NCHW assertion + import opencv.dnn; on macos-arm64.

[Sunrisepeak]

Upgraded: windows now has dnn too. The windows full+videoio+dnn snapshot succeeded — dnn + vendored protobuf compile clean under clang-cl (cap_ffmpeg: YES, 309 dnn TUs, 0 errors). Descriptor now carries videoio and dnn on all three platforms (x86 delta on linux/windows, NEON on macOS). dnn tests widened to windows as well.