nx_tls13_server_session_emit_sf.nx

nx_tls13_server_session_emit_sf.nx -- 7th TLS server stone. Emits the server Finished handshake message, AEAD-encrypted as a TLS 1.3 record under server_hs_traffic_key + iv. Per RFC 8446 §4.4.4: finished_key = HKDF-Expand-Label(traffic_secret, "finished", "", hash_len) verify_data = HMAC-SHA256(finished_key, transcript_hash) Where transcript_hash is the hash of all handshake messages up to (but not including) this Finished. Inner content (handshake plaintext): HT_FINISHED = 0x14 (1 byte) body_len = 0x000020 (3 bytes, u24 big-endian) verify_data (32 bytes for SHA-256) Total = 36 bytes Record format: same as emit_ee -- header(5) + ct(36+1) + tag(16). Precondition: state == CERT_SENT (set by emit_ee; we're skipping the Certificate + CertificateVerify stones for now -- substrate- honest about a stub server that doesn't authenticate to the client cryptographically yet). Postcondition: state == SF_SENT. SUBSTRATE HONESTY NOTE: a production server MUST emit Certificate + CertificateVerify before Finished, signed with the server's private key. This commit ships Finished EARLY (skipping cert + verify) so the AEAD record + transcript-finished pipeline is exercised end-to-end. A real client would refuse this handshake because the server is anonymous. Stones 6.5 + 7.5 (queued) ship the cert + verify messages. license_tier: ORIGINAL