CultCache mints storedAt from an internal Clock (default the wall clock); the vector writer pins it, so
union-surface.cc is regenerated in process and byte-compared on every run. The gate test sets the switch
itself and writes only a probe, so it neither skips nor touches a committed vector.
This commit does not pass: union-surface.cc still holds the old wall-clock stored-at; the next commit
regenerates it. Builds.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>