Submit assessment findings for Core to validate and sign

Organization API key with decision:write (role MEMBER or higher); the organization comes from the key. Core checks record digests, as-of provenance, assessed and total counts, coverage and partial status, and transaction-time amounts in each record's own currency, recomputes the per-currency totals and signs the artifact (decionis.assessments.findings/1) with the dossier Ed25519 key, verifiable offline against /.well-known/decision-dossier-jwks.json. Without a signing key the artifact is stored and labelled unsigned. A partial assessment stays partial. An identical resubmission returns the artifact signed the first time (200).