Submit the signer's field values and signature

The signing action (spec §5.5): validates ownership and required-field completeness, persists values, appends participant.signed with audit facts, and advances to the next participant — or, when this was the final signer, triggers artifact generation (the envelope stays in_progress until artifacts exist; §4 status never lies). Replaying after success is an idempotent 200. Requires prior consent.