Document case-sensitive ARN ID collision design

[InfoSecHack]

Summary

• documents the confirmed case-sensitive ARN node_id collision caused by canonical_id lowercasing all string fields
• records edge_id impact risk and separate constraint_id review requirement
• proposes a field-aware v3 ID algorithm/migration plan before any code behavior change
• lists required tests and downstream ARF-RT/probe-overlay/finding-diff compatibility risks

Validation

• grep -nEi canonical_id/lowercase/node_id/edge_id/constraint_id/ARF-RT/migration/next-slice docs/specs/case-sensitive-arn-id-collision-design.md
• grep account/ARN hygiene scans under docs/tests: clean
• artifact hygiene scan: clean
• ./scripts/check.sh
• ./scripts/test_fast.sh
• git diff --check

Scope

Docs/spec only. No production code changes, no ID algorithm changes, no live AWS, no Terraform.