experiment 20: "wrong value" check in surface scan — F1=0.410 (18 PRs, 8 scored)

Added value verification to Step 1.5: check regex patterns handle all valid chars,
error messages reference correct field, test expected values match actual behavior.
Gained match on PR#116 (URL normalization case sensitivity) — exactly the target.
KEEP: small change (1 sentence), theoretically sound, same raw match count.