test(storage): add exact V4 migration fixture

[hubab1]

Summary

• add a deterministic store fixture generated by the exact released V4 source tree
• validate fixture provenance, manifest hashes, all 21 V4 entity types, and representative difficulty evidence
• prove injected open failures preserve the copied SQLite store and sidecars byte-for-byte
• generalize the strict released-store fixture loader without weakening the existing V1 checks

Current-main restack

The original single commit was reconstructed on current main at 7bdbca2. The refreshed PR contains only the intended ten fixture/test files and no product, schema, or UI changes.

Validation

• exact V4 fixture and persistence migration suites: 10 tests passed
• full project suite: 472 tests passed
• fixture SHA-256 values match the committed manifest
• fixture-generator shell syntax passed
• Xcode project validation and git diff --check passed
• no application launch or live user-store access

Review gate

Draft/manual review remains appropriate because this fixture is the migration artifact required before any future schema revision.

Contributes to #3.