Add public reviewer evidence guide

[InfoSecHack]

Summary

• Adds docs/REVIEWER_GUIDE.md as a concise evidence map for reviewers.
• Links the reviewer guide from docs/START_HERE.md.
• Keeps claims bounded to the local synthetic path-overcounting demo and one two-sided controlled PassRole-to-Lambda validation pair.

Validation

• reviewer-guide targeted grep: passed
• START_HERE link grep: passed
• account/ARN/artifact hygiene scans: no output
• ./scripts/check.sh: passed
• ./scripts/test_fast.sh: 1845 passed
• git diff --check: passed