Validators on generic Proof of Stake chains earn the same fees whether they handle attestation work correctly or selectively censor it. For chains whose main activity is moving tokens around, that indifference is fine. For chains whose primary economic activity is recording attestations (content provenance, AI-output attribution, threshold-signed credentials, supply-chain receipts), the indifference becomes a problem. Proof of Useful Attestation (PoUA) makes attestation handling first-class in the consensus weighting itself. Validator vote weight is the product of bonded stake and a reputation scalar in [r_min, r_max] that accumulates from valid attestation work. The reputation update is additive, fee-weighted, non-transferable, and capped per epoch. We prove a cost-to-grind floor (Lemma 1): under chain-wide adaptive burn fraction tau_burn, the non-recoverable cost an adversary pays to inflate reputation by Delta_r is bounded below by tau_burn * Delta_r / (eta * alpha_eff). Under the recommended v0 calibration (r_max/r_min in [4, 10]), the cost premium against a capital adversary is 4x to 10x over equivalent pure-stake PoS at steady state. The paper specifies the mechanism, six layered Sybil and grinding defenses, empirical Monte Carlo strategy-search across the full layered defense, and grinding detectors with explicit threshold derivations. It is a mechanism-design proposal with a formal economic floor and inherited BFT safety and liveness, not a complete cryptographic security proof. This release incorporates feedback from Jiangshan Yu (University of Sydney) and Marko Vukolić (Bitcoin Scaling Labs).