For agents, on top of README.md, which they read first.

• Rust builds at -Copt-level=3 for every target; the reason is in the comment in BUCK. It is a property of the project, not a release mode.
• Builds outside nix develop pick up whatever rustc the host has, or none. Always build through the devshell.
• buck2's system cxx toolchain looks up plain clang/clang++ and links with -fuse-ld=lld; both come from the devshell (../flake.nix says why). Without them every Rust binary and build script fails at the link.