feat(storage): add research project schema

[hubab1]

Summary

• add append-only schema V5 with research-project and project-keyword membership models
• preserve released V1 through V4 schemas and use a lightweight V4-to-V5 migration
• keep query, ranking, metric, and difficulty evidence shared when a project is deleted
• use independent project UUIDs rather than fake App Store IDs
• make project and membership identity immutable and rotate incarnation UUIDs after delete/recreate

Current-main restack

The schema commit was reconstructed on current main after the exact V4 fixture merged. The refreshed PR contains one conventional commit and only the intended schema, migration, and persistence-test files.

Validation

• focused research-persistence, migration, and exact-fixture suites passed
• full project suite: 479 tests passed
• Swift 6.2 complete strict-concurrency build passed
• Xcode project validation and git diff --check passed
• no application launch or live user-store access

Review gate

This changes the persistent schema from V4 to V5. It remains a separate PR so the migration can be reverted or diagnosed independently of project lifecycle and UI work.

Contributes to #3.