Add external reviewer start here

[InfoSecHack]

Summary

• Add a concise external reviewer start-here page for the corrected prod-like v3 evidence slice.
• Link the evidence brief and corrected checkpoint.
• Preserve bounded wording, evidence categories, and explicit non-claims.

Validation

• Targeted grep for core numbers and non-claims: passed
• Account/ARN hygiene scans: clean
• Terraform/raw artifact scan: clean
• ./scripts/check.sh: passed
• ./scripts/test_fast.sh: 1999 passed
• git diff --check: passed