LRU chunk eviction (bounded cache, default 1024 chunks / ~200MB)

The in-memory chunk+frame cache no longer grows unbounded as players
explore. An LRU policy (doubly-linked list + index map, O(1) per op)
evicts least-recently-used chunks when the cache exceeds MaxCachedChunks,
dropping both the chunk and its cached frame. Dirty chunks are skipped
until the autosave flushes them, so no edit is ever lost to eviction.

- world/cache.go: maxChunks field + order/index LRU bookkeeping; touch
  (move-to-front) on every chunkAt/Frame/SetBlock hit; evictIfNeeded on
  miss; NewCacheWithLimit constructor (0 = unbounded, backward-compat).
  Dirty chunks are bumped to MRU and left in place rather than doing
  region I/O under the cache mutex.
- server/server.go: Config.MaxCachedChunks (default 1024); New wires it
  into NewCacheWithLimit when a world dir is set.
- cmd/regionio/main.go: -maxcache flag.
- Tests: limit cap, LRU ordering (touched chunk survives), both-maps
  drop, dirty-keep, reload-on-access, and edits-survive-eviction+reload
  (end-to-end via the store).

cmd/regionio/main.go

@@ -28,10 +28,12 @@ func main() {
 	seedFlag := flag.Int64("seed", parseSeedEnv(os.Getenv("REGIONIO_SEED"), cfg.WorldSeed, log),
 		"world seed (overrides REGIONIO_SEED)")
 	worldDir := flag.String("world", cfg.WorldDir, "world directory (empty = in-memory only)")
+	maxCache := flag.Int("maxcache", cfg.MaxCachedChunks, "max cached chunks, LRU eviction (0 = unbounded)")
 	flag.Parse()
 	cfg.WorldSeed = *seedFlag
 	cfg.WorldDir = *worldDir
-	log.Info("using world seed", "seed", cfg.WorldSeed, "worldDir", cfg.WorldDir)
+	cfg.MaxCachedChunks = *maxCache
+	log.Info("using world seed", "seed", cfg.WorldSeed, "worldDir", cfg.WorldDir, "maxcache", cfg.MaxCachedChunks)
 
 	srv, err := server.New(cfg)
 	if err != nil {