Directory.Build.props names CultLibRoot (CULTLIB_ROOT, else the sibling
checkout) and pins CultLibRevision to f0605367 (CultMath 0.2.1 on main).
Directory.Build.targets carries Delvehold's VerifyCultLibRevision guard,
checking for CultMath.csproj: HEAD must equal the pin and the worktree must
be clean.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>