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
- Source commit: 118e60a905878e56dc4f9c5b309143c5f447c702
- Platform: macOS arm64 static executable
- Minimum macOS version: 14.0
- Deterministic install prefix: `/opt/bd-to-avp/gpac`
- Build toolchain: Xcode 27.0 (27A5194q), Apple clang 21.0.0
- Executable SHA256: 61ddca2713e5de02798820e9e295edc959d09a8b15a30dcacc3d9c14df627a6a

The vendored build is a static MP4Box executable targeting macOS 14.0, 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`, verifies the pinned GPAC commit and tag from a
clean checkout, builds MP4Box, verifies arm64/system-only linkage, and installs the executable into
`bd_to_avp/bin`. It also verifies the pinned minimum macOS version before
accepting the binary.
