Initial commit: RegionIO Minecraft server core (26.1.2/protocol 775)

Vanilla-faithful overworld generator (final_density + multi-noise biomes),
full connection lifecycle (status/login/configuration/play), chunk streaming,
creative block editing, and the protocol/nbt/registry infrastructure.

internal/world/chunk.go

@@ -0,0 +1,23 @@
+// Package world provides chunk data for the play phase: an in-memory chunk
+// representation, the level_chunk_with_light encoder, and (for now) a flat
+// world generator. Real noise-based generation arrives in a later sub-milestone.
+package world
+
+// FlatSurfaceY is the Y of the topmost solid block (grass) in the flat world.
+// A player spawns one block above it.
+const FlatSurfaceY = -61
+
+// GenerateFlat builds a Classic-Flat-style chunk at (cx, cz): bedrock at the
+// world floor, two dirt layers, and a grass surface, all under a plains biome.
+func GenerateFlat(cx, cz int32) *Chunk {
+	c := NewChunk(cx, cz, BiomePlains)
+	for lx := 0; lx < 16; lx++ {
+		for lz := 0; lz < 16; lz++ {
+			c.SetBlock(lx, MinY+0, lz, StateBedrock)
+			c.SetBlock(lx, MinY+1, lz, StateDirt)
+			c.SetBlock(lx, MinY+2, lz, StateDirt)
+			c.SetBlock(lx, FlatSurfaceY, lz, StateGrass)
+		}
+	}
+	return c
+}