docs: add domain map and improve onboarding

[vinzlercodes]

Summary

Describe the user-visible or contract-visible change.

Verification

• Added or updated a behavior test.
• Ran the relevant checks from CONTRIBUTING.md.
• Used synthetic data only.
• Updated public documentation or the OpenAPI contract when needed.

Safety

• This change preserves immutable snapshots, separate CBB and IRC calculations, and human approval.
• This pull request contains no credentials, real financial data, sensitive documents, or precise geolocation.