feat(stdlib): add SHA-256, MD5, HMAC-SHA256, ConstantTimeEqual to Security.Crypto
Expand the crypto primitives unit with four new capabilities:
- SHA-256 / Sha256Hex (FIPS 180-4, Rotr32 helper, 64-round K table)
- MD5 / Md5Hex (RFC 1321, little-endian padding and output, T/S tables)
- HMAC-SHA256 / HmacSha256Hex (RFC 2104, key>block-size hashing)
- ConstantTimeEqual (timing-safe XOR-accumulate comparison)
Also refactors Sha1Hex to use a shared DigestToHex private helper,
adds Rotr32 alongside Rotl32, removes unused `uses Classes` import,
and adds MD5 deprecation notice to the unit header.
Tests: 12 crypto tests covering NIST CAVP (SHA-256), RFC 1321 all 7
vectors (MD5), RFC 4231 TC1-3/TC5-6 (HMAC-SHA256 incl. long-key),
and 8 ConstantTimeEqual assertions. E2E smoke test for Sha256Hex
on both backends (cp.test.e2e.crypto.pas).
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