Root cause of the test-fast failures (confirmed via a pure-main probe PR that
failed identically): the fast job builds with -j$(nproc) inside an 800m-memory
container, and parallel Eigen+GoogleTest compilation OOM-kills cc1plus
("Killed signal terminated program cc1plus") non-deterministically — unrelated
to branch content (only constants.hpp reaches the fast target's header closure
on this branch, with trivial constexpr additions).
Serialise the fast build (-j1), mirroring the Pi-protection already applied to
the test-cgal job. Fixes test-fast on main and on this PR.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>