Avoid recomputing HMAC digests during comparison

[OskarEichler]

Summary

Snapshot each HMAC digest once in HMAC#== and reuse those strings for the size check and constant-time comparison.

Verification

• Current candidate: 630 tests / 4,624 assertions, zero failures/errors, two expected FIPS omissions
• Release candidate: 597 tests / 4,411 assertions, zero failures/errors, two expected FIPS omissions
• 100,000 comparisons: allocations 400,018 to 200,018; elapsed 0.215s to 0.099s locally
• Native build succeeds with -Werror; gem build/install and Songstats HMAC integration pass

Comparison semantics and the constant-time primitive are unchanged; this only avoids two redundant digest snapshots.