feat: run the music server on macOS (Metal, unified memory)

[stalexxx]

Summary

• Non-Windows builds skip the VC++ runtime check (nothing to fetch).
• macOS hardware detection: chip name from system_profiler (sppci_model), unified memory reported as total RAM.
• Auto backend on macOS lets the engine pick its device (Metal), never Vulkan; falls back to CPU if that start fails.
• Windows-only tests gated with #[cfg(windows)]; device_chain test is platform-aware.

Windows behaviour is unchanged.

Test plan

• cargo test -p music-server --lib on macOS (281 passed, 4 ignored)
• Windows CI / regression check

🤖 Generated with Claude Code

[stalexxx]

Besides the macOS support in music-server, this PR now includes the engine and app builds for macOS (Apple Silicon):

• scripts/build-yue-runtime.sh + a macos entry in engines/yue2-cpp-source.json: builds the pinned yue2.cpp commit with Metal (see README, "Building from source").
• scripts/build-release-macos.sh: builds an ad-hoc-signed .dmg with the engine bundled. The engine is looked up in Contents/Resources, data goes to ~/Library/Application Support/YuE2 Studio.

Built from scratch on Apple Silicon: the engine builds and starts, the dmg builds (53 MB), its signature verifies, and cargo test -p music-server --lib passes (281). The dmg was installed and run on an M2 Max: the engine from Contents/Resources starts, engine.log shows the Metal device (MTL0 (Apple M2 Max), unified memory), and a song was generated end to end (39.4 s of audio in 108.6 s) and saved to the library.

Audio to MIDI (ace-midi) is built with Metal by scripts/build-midi-runtime.sh and bundled in the dmg; transcribing a generated song on the M2 Max works (476 notes in ~30 s).

Not verified: Windows (behaviour unchanged by design).