MP4Box macOS arm64 binary

BD_to_AVP packages MP4Box as a separate command-line executable for final
QuickTime-compatible MV-HEVC, audio, and tx3g subtitle muxing.

Vendored binary source:
- Project: GPAC / MP4Box
- Version: 26.02.0
- Source tag: https://github.com/gpac/gpac/tree/v26.02.0
- Platform: macOS arm64 static executable
- Executable SHA256: eb63ed3f3c73eadc61f9158961114a614c208cd5929f1abb7d13da3a679cfedf

The vendored build is a static MP4Box executable built from GPAC source with
Homebrew hidden from the build environment and external codec, image, network,
and SSL libraries disabled. The resulting executable links only to macOS system
libraries (`/usr/lib/libz.1.dylib` and `/usr/lib/libSystem.B.dylib`).

MP4Box / GPAC is distributed under LGPL-2.1-or-later. BD_to_AVP invokes MP4Box
as a separate command-line executable. Downstream redistributors must preserve
GPAC's applicable license obligations and corresponding-source access.

Embedded build configuration reported by `MP4Box -version`:

--static-bin --disable-rmtws --disable-network --disable-ssl --disable-x11 --disable-sdl --use-zlib=system --use-opensvc=no --use-openhevc=no --use-platinum=no --use-freetype=no --use-ssl=no --use-jpeg=no --use-openjpeg=no --use-png=no --use-mad=no --use-a52=no --use-xvid=no --use-faad=no --use-ffmpeg=no --use-freenect=no --use-vorbis=no --use-theora=no --use-nghttp2=no --use-ngtcp2=no --use-nghttp3=no --use-lzma=no --use-curl=no

GPAC source and license information:
- Source: https://github.com/gpac/gpac
- Releases: https://github.com/gpac/gpac/releases
- Upstream license file: https://github.com/gpac/gpac/blob/master/COPYING
- GNU LGPL v2.1 text: https://www.gnu.org/licenses/old-licenses/lgpl-2.1.txt

The build script is `scripts/build_mp4box_macos.py`. It reads
`vendor/mp4box-macos-arm64.toml`, checks out the pinned GPAC tag, builds MP4Box,
verifies arm64/system-only linkage, and installs the executable into
`bd_to_avp/bin`.
