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, every V4 entity type, and representative difficulty evidence
• prove 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

Validation

• fixture generator: 1/1 passed
• exact V4 foundation tests: 4/4 passed
• full test suite: 336/336 passed
• signed Debug build passed; code signature verified
• no application launch or live user store access

Review gate

Draft/manual review required because this establishes the migration artifact used by the next schema revision.

Contributes to #3.