Composite signatures for PQC migration

A composite signature carries two components — e.g. Ed25519 and ML-DSA-65 — over the same payload in one verifiable envelope (COSE-encoded). During migration:

1. Classical-only verifiers keep working (they check the component they know).
2. Hybrid verifiers require both.
3. When the classical algorithm is retired, hybrid keys downgrade gracefully to the post-quantum component per your policy — the registry’s deprecated/retired statuses drive this.

Crates: confium-composite (envelopes + verifiers) and confium-composite::transition_verifier (policy-driven checking).