perf: avoid unused legacy serialization and unit formatting

[maebli]

Legacy JSON and YAML rendering now serialize only the requested format; wide CSV and summary-only consumers skip both serializations. Canonical unit formatting generates its display fallback only when UCUM conversion fails, avoiding unused formatting and a metadata traversal when conversion succeeds.

Validation:

• All 700 captured outputs across 100 fixtures and seven formats match byte-for-byte.
• cargo test --all-features and cargo test --features std pass.
• Library Clippy with all features, formatting, and diff checks pass.
• Added coverage for mixed supported/unsupported units and empty units.

Root all-target Clippy remains blocked by existing warnings in unrelated annotation and integration tests.