Module tiers 

AOI three-layer memory tier promotion.

Provides a background sweep loop that promotes frequently-accessed episodic messages to the semantic tier by:

1. Finding candidates with session_count >= promotion_min_sessions.
2. Grouping near-duplicate candidates by cosine similarity (greedy nearest-neighbor).
3. For each cluster with >= 2 messages, calling the LLM to distill a merged fact.
4. Validating the merge output (non-empty, similarity >= 0.7 to at least one original).
5. Inserting the semantic fact and soft-deleting the originals.

The sweep respects a CancellationToken for graceful shutdown, following the same pattern as eviction.rs.

Structs

TierPromotionConfig

Configuration for the tier promotion sweep, passed from zeph-config::TierPromotionConfig.

Functions

start_tier_promotion_loop

Start the background tier promotion loop.