TASK: Create the data_connect package as a mockable abstraction layer.

You must:
1. Define abstract repositories for each domain group
2. Create fake/mock implementations using in-memory data
3. Simulate async GraphQL-style behavior
4. Ensure replaceability with real generated SDK later

Rules:
- No Firebase imports
- No HTTP
- No direct entity mutation
- Return domain entities ONLY

Must follow archtiecture principles defined in:
- docs/01-architecture-principles.md

Must Follow Agent rules defined in:
- docs/02-agent-development-rules.md

Include:
- Interfaces
- Fake implementations
- Clear TODO markers for real SDK replacement

This package must compile and be dependency-safe.