test(insights): protect rolling bitmap semantics

[olalie]

Summary

Add comprehensive regression coverage for the rolling 30-day bitmap semantics in insights processing.
Clarify the internal anchor-day semantics used for bitmap interpretation without changing persisted JSON compatibility.

Changes

• add regression coverage for bitmap aging and pruning
• add boundary tests for delta=29/30/31 behaviour
• add tests for same-day additive snapshot semantics
• add tests for pruning and re-observation behaviour
• verify inclusive tie-breaking behaviour in top lists
• clarify l semantics as bitmap anchor day
• use anchor_day terminology internally where helpful
• tighten contract-oriented docstrings and comments

Notes

• no runtime behaviour changes
• no persistence-format changes
• persisted JSON field "l" remains unchanged
• internal semantics clarified only

Testing

• pytest
• 237 passed