Chunk persistence to Anvil .mca region files

The world now survives restarts: chunks load from disk (read-through
cache) and player edits persist via async autosave + a final SaveAll on
shutdown. RegionIO finally does region I/O.

- world/regionfile.go: Anvil .mca container — 8192-byte header
  (offset + timestamp tables), 4096-byte sectors, zlib chunk records.
- world/compress.go: zlib deflate/inflate for chunk payloads.
- world/store.go: chunk <-> Level-nested NBT (per-section
  block_states/biomes palettes, WORLD_SURFACE heightmap, DataVersion
  4790, yPos -4) via the existing nbt package; Store opens one
  RegionFile per region with proper floor-division coords.
- world/state_names.go: id->name bridge from the embedded blocks.json
  report so network int-IDs round-trip through the disk named palette.
- world/encode.go: GetBiome read accessor for serialization.
- world/cache.go: read-through (disk then generation), dirty tracking,
  StartAutosave (returns a done channel so the saver exits before
  Close), SaveAll, NewCacheWithStore.
- server.go + main.go: Config.WorldDir (default "world"), -world flag,
  autosave loop every 30s, SaveAll + store Close on signal.
- Tests: region round-trip/absent/overwrite, chunk NBT round-trip,
  end-to-end save-reload, negative chunk coords, autosave persistence.

internal/server/server.go

@@ -20,6 +20,10 @@ type Config struct {
 	CompressionThreshold int
 	// WorldSeed seeds terrain generation.
 	WorldSeed int64
+	// WorldDir is the on-disk world directory. When non-empty, chunks are
+	// read from and written to <WorldDir>/region/*.mca and player edits
+	// survive restarts. Empty disables persistence (in-memory only).
+	WorldDir string
 }
 
 // DefaultConfig returns sensible defaults matching vanilla expectations.
@@ -33,6 +37,9 @@ func DefaultConfig() Config {
 		// WorldSeed defaults to 0 for backward compatibility; operators override
 		// it via the REGIONIO_SEED env var or the -seed flag.
 		WorldSeed: 0,
+		// WorldDir defaults to "world" so the world persists by default;
+		// set to "" for a throwaway in-memory world.
+		WorldDir: "world",
 	}
 }
 
@@ -40,14 +47,26 @@ func DefaultConfig() Config {
 type Server struct {
 	cfg    Config
 	chunks *world.Cache
+	store  *world.Store // nil when persistence is disabled
 }
 
-// New constructs a Server from cfg.
-func New(cfg Config) *Server {
+// New constructs a Server from cfg. When cfg.WorldDir is set, the world is
+// backed by an on-disk store under that directory; otherwise it is in-memory
+// only. A returned error (e.g. the world dir cannot be created) is fatal.
+func New(cfg Config) (*Server, error) {
+	gen := world.NewVanillaGenerator(cfg.WorldSeed)
+	if cfg.WorldDir == "" {
+		return &Server{cfg: cfg, chunks: world.NewCache(int32(cfg.CompressionThreshold), gen)}, nil
+	}
+	store, err := world.NewStore(cfg.WorldDir)
+	if err != nil {
+		return nil, err
+	}
 	return &Server{
 		cfg:    cfg,
-		chunks: world.NewCache(int32(cfg.CompressionThreshold), world.NewVanillaGenerator(cfg.WorldSeed)),
-	}
+		chunks: world.NewCacheWithStore(int32(cfg.CompressionThreshold), gen, store),
+		store:  store,
+	}, nil
 }
 
 // Config returns the active configuration.
@@ -56,6 +75,9 @@ func (s *Server) Config() Config { return s.cfg }
 // Chunks returns the shared chunk cache.
 func (s *Server) Chunks() *world.Cache { return s.chunks }
 
+// Store returns the on-disk world store, or nil if persistence is disabled.
+func (s *Server) Store() *world.Store { return s.store }
+
 // statusResponse mirrors the JSON shape the client expects for the server-list
 // ping. Field names and nesting are part of the protocol contract.
 type statusResponse struct {