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.
