AttestPage

Use when an agent needs to show later that a file, answer or record existed unchanged at a point in time: send its sha256, and get back a signed, timestamped receipt over it; a later altered copy no longer matches. Optional note; with an idempotency_key (x402 payments only), a retry gets the same receipt and is not a new record. For signed evidence of what a web page said at a given time (provenance), use /v1/fetch, whose receipt attests the page hashes.