hmac_ada

SPARK-proved HMAC (RFC 2104) with standalone SHA-256 (FIPS 180-4) for Ada 2022. 175 proof obligations fully discharged at Level 2 with zero pragma Assume. Constant-time digest comparison (the default = operator), secure wipe of key material, no heap allocation, pragma Pure. Suitable for embedded and safety-critical systems. Includes streaming and one-shot APIs, plus a generic HMAC package for other hash functions.