feat(02): GREEN — argon2id Hash + Verify + self-test
- Add Params struct with Memory/Iterations/Parallelism/SaltLength/KeyLength
- DefaultParams: OWASP 2024 baseline (m=64KiB, t=1, p=4, salt=16B, key=32B) — D-08
- TestParams: reduced cost (m=8KiB) so go test stays under 5s — D-26/Pitfall 4
- Hash(): crypto/rand salt per call, argon2.IDKey, PHC format $argon2id$v=19$...
- Verify(): PHC split/parse, ErrInvalidHash on malformed, ErrIncompatibleVersion on v!=19
- subtle.ConstantTimeCompare for timing-attack resistance (T-2-13)
- init() self-test: hash/verify round-trip panics on regression (D-08/T-2-15)
- Add golang.org/x/crypto v0.51.0 as direct dependency
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