Cache at ~/.gstack/eval-cache/{suite}/{sha}.json. Compute cache keys
from source file contents + test input via Bun.CryptoHasher SHA256.
Supports read/write/stats/clear/verify operations. EVAL_CACHE=0
skips reads for force-rerun. 16 tests including corrupt JSON handling.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>