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/light.go

@@ -0,0 +1,45 @@
+package world
+
+import "regionio/internal/protocol"
+
+// lightSections is the number of light subchunks: one below the world and one
+// above, plus one per block section.
+const lightSections = SectionCount + 2
+
+// writeLight emits a fully-lit sky: every light section carries sky light 15,
+// and block light is reported as uniformly empty. This avoids a black world
+// without implementing real light propagation (deferred).
+func (c *Chunk) writeLight(w *protocol.Writer) {
+	full := allSectionsMask()
+
+	writeBitSet(w, full)              // sky light mask: all sections present
+	writeBitSet(w, nil)               // block light mask: none present
+	writeBitSet(w, nil)               // empty sky light mask: none empty
+	writeBitSet(w, full)              // empty block light mask: all empty
+
+	// Sky light arrays: one 2048-byte (4096 nibbles) array of 0x0F per section.
+	bright := make([]byte, 2048)
+	for i := range bright {
+		bright[i] = 0xFF // two nibbles of 15
+	}
+	w.VarInt(lightSections)
+	for i := 0; i < lightSections; i++ {
+		w.VarInt(2048)
+		w.Raw(bright)
+	}
+
+	w.VarInt(0) // no block light arrays
+}
+
+// allSectionsMask returns a bitset (as longs) with the low lightSections bits set.
+func allSectionsMask() []uint64 {
+	return []uint64{(uint64(1) << lightSections) - 1}
+}
+
+// writeBitSet emits a length-prefixed array of longs.
+func writeBitSet(w *protocol.Writer, longs []uint64) {
+	w.VarInt(int32(len(longs)))
+	for _, v := range longs {
+		w.Int64(int64(v))
+	}
+}