Add small in-actor LRU cache for hot assets

AssetStoreServer runs on a single dedicated smarm actor thread, so a
plain HashMap+VecDeque LRU in front of the SQLite lookup needs no
locking. Capacity configurable via CCC_CACHE_CAPACITY (default 256).

Bench (bench/RPS): +42-47% RPS under a cache-sized/hot-set workload,
but a small net loss under adversarial uniform-random access with a
cache smaller than the catalog. Real traffic is hot-set skewed, so
net win in practice; capacity should be tuned to the expected hot set.

README.md

@@ -27,6 +27,8 @@ docker compose exec ccc /usr/local/bin/ccc add demo /staging/app.js 1.0.0
 (Alternatively `docker cp` a file straight into the container if you'd
 rather not use the staging mount.)
 
+_See [bench/RPS](bench/RPS) for an RPS bench of the store's LRU cache vs. raw SQLite._
+
 ## License
 
 AGPL-3.0-only - see [LICENSE](LICENSE). If you run a modified version