Two blockers fixed in .cargo/config.toml plus a documented gate script:
- target-cpu=x86-64-v3 (AVX2-only) is now opt-in — the default build is
portable; on Sandy Bridge (x86-64-v2) it miscompiled build scripts
(num-traits SIGABRT, 648518346341351440-byte allocation).
- AR=llvm-ar dropped (GNU ar is fine); CC=clang/CXX=clang++ retained
because build.rs needs clang for -flto=thin, resolved via PATH.
scripts/gate.sh records the review-box command: init submodules, wire the
user-local LLVM-18 / pkgconf / fontconfig-X11 roots (no sudo available),
then cargo check --all-targets. docs/build-gate.md explains the failure
modes. Verified: ./scripts/gate.sh exits 0 end-to-end on this box.
