Agent-proof codebase

Make acceptance mechanically expensive to fake, so whatever gets merged — from a human, a weak agent, or a strong one — is high-signal: it fixes a real defect, improves a measured property, or clears a proven edge case, with machine-checked evidence attached.

This is not "write more tests". A million weak tests are worth less than a hundred that fail when the code is wrong. For every gate ask: what is the cheapest way to pass without being correct, and have I closed it?