feat(storage): add research project schema

[hubab1]

Summary

• add append-only schema V5 with research-project and project-keyword membership models
• preserve the released V1 through V4 schemas and use a lightweight V4 to V5 migration
• keep query, ranking, metric, and difficulty records shared when a project is deleted
• make project and membership identity/canonical scope immutable after construction
• add durable, unique incarnation UUIDs so delete/recreate cannot make stale handles current again
• cover exact-fixture and synthetic migration, relationship, generation, and cascade behavior

Validation

• focused persistence, migration, and dependency tests: 35/35 passed
• full test suite: 343/343 passed
• signed Debug build passed; Apple Development signature verified
• independent P0/P1/P2 review clear after the incarnation-token fix
• no application launch or live user store access

Review gate

Draft/manual review required because this changes the persistent schema. It is intentionally stacked on #45.

Contributes to #3.