Produce a composite signature

confium pki composite-sign \
  --message @document.bin \
  --ed25519-key ed.hex \
  --p256-key ec.pem \
  --out composite.hex

• --message accepts a literal or @file for exact file bytes.
• The envelope is COSE-encoded; both components cover the same payload, so classical-only verifiers keep working during migration (see composite signatures).
• Verify with confium verify composite --message ... --signature composite.hex --algorithm ed25519 --public-key pk.hex.