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.

.gitignore

@@ -3,3 +3,7 @@
 
 /staging/*
 !/staging/.gitkeep
+
+# bench harness generates these locally (see bench/RPS); not meant to be committed
+bench/urls*.txt
+bench/full_urls*.txt