docs: enhance Rust test framework POC plan with review recommendations

Improvements based on plan review:

- Add Task 0.5: Verify prerequisites (Rust, mise, Docker, psql)
- Add Success Criteria section with clear metrics and rollback strategy
- Improve Task 3, Step 3: Add function signature verification for selectors
- Enhance Task 12: Better error messages in Rust helper with unwrap_or_else
- Document Task 4: Add fixture dependency comments (array_data → encrypted_json)
- Add test coverage metric: 16 tests from 2 source files

These changes improve plan clarity, verification steps, and error handling
while maintaining the POC scope.

Author: tobyhede