Submit a signed chain checkpoint to the witness

Records an Ed25519-signed chain checkpoint and returns a receipt signed by Headless Oracle stating when it was received. The receipt attests only that at received_at the witness was shown a checkpoint validly signed by the key with that thumbprint; it does not attest who controls the key or that the records are true. Checks run in order and fail closed with 400 on the first failure: bad_request, bad_checkpoint_shape, bad_checkpoint_field, bad_jwk, kid_mismatch, bad_signature. checkpoint.sig is base64url without padding (86 chars), never hex. A checkpoint's identity is (kid, session_id, count, last_entry_hash): a new identity returns 201, a stored one returns 200 with the stored receipt. About 60 requests per minute per client address, counted per Cloudflare location; 2,000 new anonymous rows per UTC day, best effort. Optional Authorization: Bearer <Evidence plan key> counts against the key's own quota instead (up to 1,000 or 3,000 new checkpoints per UTC day) and is limited at about 600 per minute per address and per account. Full contract and honest limits: GET /v1/witness/spec.