fix(crypto)!: fingerprint, sid, and verify owners

[sephynox]

Summary

Finishes typed certificate fingerprints and SKIDs, forces low-s on every verify path that already holds a prehash, and puts mux accept/authorize on one negotiation carrier. Callers can no longer skip SID checks or mint fingerprints from raw digests.

Related Issues

None

Changes Made

• Mint fingerprints only via Fingerprint::from_certificate
• Require low-s on CMS, ECIES, frame, and receipt verify
• Put mux accept and authorize on TransportNegotiation
• Share expiry checks through validate_expiry_at
• Drop unused CryptoPolicyError, apply_domain, and Secret FixedTag

Testing

• make test-all: passes

Breaking Changes

• Fingerprint / Skid are newtypes. Replace [u8; 32] / [u8; 20] aliases
• Const pin/deny lists take &'static [u8; 32]. Runtime pins take Fingerprint<D>
• CryptoPolicyError and apply_domain are gone. Map rejects to CertificateValidationError
• Signature types used with SigningProvider / Frame::verify must implement LowSEncoding

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Note

High Risk
Touches authentication-critical paths (signature verification, certificate trust/pinning, handshake receipts, and peer identity) with breaking API changes; incorrect migration could weaken identity binding or accept malleable signatures.

Overview
Introduces typed Fingerprint<D> and Skid for certificate identity, routes trust/pinning/revocation and cluster peer keys through Fingerprint::from_certificate and CertificateTrustStore::to_fingerprint::<Sha3_256>, and centralizes SKID parsing in Skid::parse.

Adds LowSEncoding and threads it through verify_canonical, SigningProvider, frame verify, CMS/ECIES handshake, and session receipts so high-s ECDSA encodings are rejected consistently. EcdsaSignatureVerifier now always requires a matching SignerIdentifier, and signer IDs are built via shared compute_signer_identifier.

Handshake mux negotiation moves onto TransportNegotiation (accept / authorize), replacing the old free functions. Certificate expiry checks share validate_expiry_at. Cleanup removes CryptoPolicyError, apply_domain, and Secret’s FixedTag; X9.63 KDF expansion is deduplicated in X963Sha3_256::expand.

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