fix: load canonical V1 COPY fixtures in integration tests

[kcrmin]

Authority: user:kcrmin

Summary

• reset backtest migration contributions for the canonical launch V1
• keep long market-data paths working on Windows
• parse pg_dump COPY FROM stdin blocks and stream them through psycopg during integration setup
• add focused loader regression tests

Verification

• focused loader tests passed
• Ruff passed
• mypy passed
• root three-lane Docker integration test passed locally