## What changed
- Track the 50 most recently invoked skills for each thread and evaluate them
as the `lru_v1` shadow-selection method.
- Filter stale and duplicate entries while preserving recency order.
- Increase the shadow result limit to 50 and add a `21_50` rank bucket.
## Testing
- Add unit coverage for recency refresh, eviction, filtering, limits, and rank
buckets.
- Add an extension test showing that `lru_v1` recovers a skill invoked on an
earlier turn.
GitOrigin-RevId: 7aa56514db57cf180cc3f57b4d493dee3304e766
