Single Source of Truth

Caveman. Deep rules + WHY each exists.

The one root cause

Every bug below is the same shape: a correctness duty got parked on human memory — "remember to update both", "remember it already ran", "remember to call the check". Humans forget; the duty decays monotonically. Every rule here moves one duty off a human and onto a mechanism that cannot forget (compiler, type, engine, derivation, test). That is the whole philosophy. Pick the mechanism per case.

A second lens: collapse the distance between a truth and its uses. Each gap — a copy, a hand-sync, a manual step, a runtime check — is a place two things can disagree. Close the gap so disagreement is impossible, not merely discouraged.