[codex] fix LongBridge signal snapshot date fallback

[Pigbibi]

Summary

• Use price_as_of / snapshot_manifest_price_as_of as the fallback signal snapshot date.
• Add regression coverage for snapshot-backed LongBridge notifications with partial history refresh metadata.

Root Cause

The notification renderer only displays signal_snapshot.market_date or signal_as_of. The snapshot builder preserved price_as_of for the input-status line but did not use it to populate the displayed snapshot date when explicit snapshot date fields were missing.

Validation

• python -m unittest tests.test_signal_snapshot
• python -m compileall -q application/signal_snapshot.py tests/test_signal_snapshot.py