Add AWS ECR registry evidence gates

[z707693052]

Closes #1204

Summary

• Add supplemental Amazon ECR container registry hardening gates to aws-review without counting them as CIS AWS Foundations controls.
• Cover tag immutability, scan-on-push/enhanced scanning, repository policy exposure, encryption, lifecycle cleanup, and rollback evidence.
• Add detailed ECR-REG checklist entries for Terraform/registry evidence.

Validation

• git diff --check
• Markdown fence balance check for both edited files
• Marker checks for Amazon ECR, image_tag_mutability, scan_on_push, and aws_ecr_registry_scanning_configuration