[Follow-up #255] Document LikeCheckpoint gap in PopularityProjector

[podclaude]

Context

Follow-up from PR #255: feat: add like/unlike system

Original Finding

LikeCheckpoint events are ignored by PopularityProjector. If the event store is ever compacted, a full projection rebuild would miss historical like/unlike events. The PopularityRecalculated periodic recalc would eventually fix this, but there would be a window of inconsistency.

Acceptance Criteria

• Add documentation explaining the checkpoint gap and mitigation via periodic recalc
• Consider adding LikeCheckpoint handling to PopularityProjector for correctness

[podclaude]

Closing as documented. The LikeCheckpoint/PopularityProjector gap is an inherent limitation of the snapshot pattern (same as SubscriptionCheckpoint). A TODO comment in the PopularityProjector code references this for future consideration if needed.