docs: add CDK validation scenario backlog

[sakul-learning]

Summary

• Add an evidence-backed backlog for turning CDK comprehensive-validation ideas into fair benchmark scenarios.
• Record the v1 decision: do not enable a CDK-only validator or online cdk validate in the current deployment-free benchmark.
• Define characterization, cross-arm-equivalence, fixture, and promotion gates; enumerate candidate SQS, Lambda, and security investigations.

Validation

• uv sync — passed.
• make build-arms && make preflight — passed for awscdk, hcl-raw, and terraconstructs under --network none.
• make check — passed: 439 passed, 1 skipped.

Sources

• https://dev.to/pahud/stop-waiting-10-minutes-to-fail-how-cdk-comprehensive-validation-catches-misconfigurations-before-1907
• https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/validate-stack-deployments.html