- signRequest now accepts optional nonce; payload becomes `${timestamp}.${nonce}.${body}` when present, falling back to `${timestamp}.${body}` for legacy peers
- verifySignature accepts matching nonce parameter and passes it through
- buildFederationHeaders generates a UUID nonce per request and includes X-Federation-Nonce header
- parseFederationHeaders extracts X-Federation-Nonce into nonce field (null when absent)