[swhitt]

What

Follow-up to #83. RasterShader builds its library from the embedded MSL blobs for every combiner/other-mode combo (rt64_raster_shader.cpp:166-167), so with source blobs I think we were paying a full newLibraryWithSource compile per specialized shader, not just once at setup.

Now RT64 checks xcrun -sdk macosx metal -v at configure. If it runs (full Xcode, CI, release builds) we precompile metallibs like before. CLT-only still embeds source, so the no-Xcode build from #83 keeps working. -DDKR_RT64_PRECOMPILE_MSL=ON/OFF overrides it. plume already handles both blob types after #83, no change there.

Also fixed the leftover Prerequisites: Xcode and Homebrew. in Build-macOS.sh, reworded the host-tools python comment, and added a note to BUILDING.md.

Pattern checklist

• First-party changes are in-place edits under runtime-recomp/, modding/ or scripts/ (no new patches/<name>/)
• Third-party changes are *.patch files registered in patches/manifest.json (amends rt64/0021 + sha256)
• Guest hooks only via guest-hooks/hooks.toml; symbols only via symbols/*.toml (n/a)
• New settings, launcher pages and owned services are registered, and their contract docs are updated (n/a)
• netplay/legacy_identity.hpp updated (reviewed by both owners) if simulation, wire or owned-service behaviour changed (n/a, renderer build only)
• Tests added or updated; python scripts/check.py --suite all passes locally
• Platforms considered: macOS only, iOS untouched. Ran the shader function in a stub harness against the real metal compiler: Xcode gives a metallib, CLT-only and =OFF give source. Haven't done a full game build with it yet.

Risk

Pretty low. Machines with the Metal toolchain go back to the pre-#83 path. The probe gets cached, so installing Xcode later needs a fresh configure (or =ON). Revert to roll back.