Server-verifiable on-chain burn, symmetric with /settlements: HMAC-signed body, single-use actionId (idempotent), bound to an action-authorization debit limit, with an optional signed `reason` slug (the grouping key) and free-form `metadata`. Verifies the supplied `txHash` is an ERC-20 transfer of exactly `amount` Project Tokens from a linked wallet to the signed `burnAddress` sink, and records it. A transfer to the dead address (0x...dead) is a real supply burn; any other sink is the app-chosen destination. `txHash` is required - burns are on-chain only.