fix: encode baseline hashes without LowerHex formatting

[pixincreate]

What

sha2 0.11.0 changes hasher.finalize() to return hybrid_array::Array instead of GenericArray, which no longer implements LowerHex for {:x} formatting. src/baseline.rs:27 used format!("{:x}", hasher.finalize()), breaking compilation (E0277) on sha2 0.11.

Fix

Use hex::encode on the digest. Works on both sha2 0.10 (GenericArray) and 0.11 (hybrid_array::Array) since both implement AsRef<[u8]>.

Verification

• cargo test with sha2 0.10.9 + hex 0.4.3: all suites pass (147 tests)
• cargo test after temporarily bumping to sha2 0.11.0: all suites pass (E0277 gone); cargo clippy clean
• Output identical to {:x} (existing hash_content_uses_expected_lowercase_hex test unchanged and passing)

Unblocks dependabot PR #80 (sha2 0.10.9 → 0.11.0).