luaperf evidence

• lu-string-concat-hot — measured (lu-string-concat-hot.md + lu-string-concat-hot.jsonl); default severity error.
• lu-print-hot — measured (lu-print-hot.md + lu-print-hot.jsonl); default severity stays hint (I/O env-dependent).
• lu-table-insert-hot — measured (lu-table-insert-hot.md + lu-table-insert-hot.jsonl); default stays off (G3: OK-idiom in OpenResty code).

See ../docs/evidence-policy.md.