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/world/encode.go

@@ -102,6 +102,21 @@ func (c *Chunk) GetBlock(lx, y, lz int) uint16 {
 	return s[blockIndex(lx, y, lz)]
 }
 
+// GetBiome returns the biome of the 4×4×4 cell containing block (lx, y, lz). It
+// mirrors GetBlock: the per-section biome array if present, else the column's
+// uniform fallback biome. Needed for on-disk chunk serialization.
+func (c *Chunk) GetBiome(lx, y, lz int) uint16 {
+	si := (y - MinY) >> 4
+	if si < 0 || si >= SectionCount {
+		return c.biome
+	}
+	b := c.biomes[si]
+	if b == nil {
+		return c.biome
+	}
+	return b[biomeIndex(lx, y, lz)]
+}
+
 // SetBlock sets the block at local (lx, lz) and absolute world height y.
 func (c *Chunk) SetBlock(lx, y, lz int, state uint16) {
 	si := (y - MinY) >> 4