Get a trader's signed track record

A trader's Ed25519-signed track record over the last 90 days, exactly as Arena signed it for the keyed audience: record is the envelope, never reshaped, so a saved copy verifies on its own. promote-record is called with audience=keyed, so the envelope has the same scores and the same signing key as the public record and names no Kalshi series ticker and no venue. It is served while the venue-data switch is off. The CLI's arena record, hosted MCP verify (get_track_record with verify: true) and the Python SDK's track_record() still read the public envelope, which names series. To check this envelope: take record without its signature field and write the rest in RFC 8785 (JCS) form as UTF-8 bytes; their SHA-256 in hex is record.signature.payload_sha256, and record.signature.sig (base64url) is an Ed25519 signature over those same bytes, not over the hash, by the key record.key_id from keysUrl. A private account answers 200 with private: true and record null, and a public account whose earlier trades are still hidden answers numbersShown: false and record null.