[rgward]

Summary

• Add a repository supplement explaining snapshot-based physical-storage seeding before a geo-replica pulls changes from the log service.
• Clarify that Azure performs seeding automatically and cross-region creation is a size-of-data operation.
• Link the supplement from the Chapter 9 overview.

Validation

• Documentation only; git diff --cached --check passed.
• Scoped to the two Chapter 9 README files; unrelated figure and tooling changes excluded.