docs: add false-positive evidence example to recipe guide

[kaizeenn]

Summary

• add one realistic false-positive example to docs/ERROR_RECIPES.md
• explain why a broad DirectX-like signal can be overconfident when evidence is partial
• add a short evidence checklist describing what makes diagnosis stronger

Linked issue

• Closes [good first issue] Add a false-positive example to the recipe guide #1

Verification

• npm run check ✅
• git diff --check ✅