feat(sha256): add output prefix adapter

[brenorb]

Summary

• add sha256_prefix for retaining a generation-time-selected digest prefix
• validate reference outputs across padding boundaries and prefix limits
• record the measured adapter cost in the README, catalog, and hash comparison

Validation

• cargo test --locked hashes::sha256::sha2_u32::tests::hashes_output_prefixes -- --nocapture
• cargo test --locked hashes::sha256::sha2_u32::tests::rejects_prefix_boundaries -- --nocapture
• cargo test --locked --test primitive_metrics
• python3 tools/kb.py validate
• cargo fmt --check
• git diff --check

The full repository test suite was intentionally not run; the focused primitive and metric checks pass.