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/network/login.go

@@ -0,0 +1,95 @@
+package network
+
+import (
+	"errors"
+
+	"regionio/internal/protocol"
+	"regionio/internal/server"
+)
+
+// handleLogin drives the offline-mode login phase:
+//
+//	C→S Login Start        → derive offline profile
+//	S→C Set Compression    → (optional) enable zlib for later packets
+//	S→C Login Success      → confirm the profile
+//	C→S Login Acknowledged → switch to the Configuration phase
+//
+// Encryption (online mode) is intentionally not implemented here.
+func (h *handler) handleLogin(pkt protocol.Packet) error {
+	switch pkt.ID {
+	case protocol.LoginStartID:
+		return h.handleLoginStart(pkt)
+	case protocol.LoginAcknowledgedID:
+		// Client acknowledges Login Success; both sides enter Configuration.
+		h.conn.SetState(protocol.StateConfiguration)
+		h.log.Info("player logged in",
+			"name", h.conn.Profile.Name,
+			"uuid", uuidString(h.conn.Profile.UUID))
+		return h.beginConfiguration()
+	default:
+		return errors.New("unexpected packet in login state")
+	}
+}
+
+func (h *handler) handleLoginStart(pkt protocol.Packet) error {
+	r := pkt.Body()
+	name, err := r.String()
+	if err != nil {
+		return err
+	}
+	if name == "" || len(name) > 16 {
+		return errors.New("invalid login name")
+	}
+	// The client also sends a UUID, but in offline mode we derive our own so it
+	// is stable and independent of what the client claims.
+	if _, err := r.UUID(); err != nil {
+		return err
+	}
+
+	h.conn.Profile = server.Profile{
+		UUID: server.OfflineUUID(name),
+		Name: name,
+	}
+
+	// Negotiate compression before Login Success so that packet (and every
+	// later one) is sent in the compressed format.
+	if t := h.srv.Config().CompressionThreshold; t >= 0 {
+		w := protocol.NewWriter(protocol.VarIntLen(int32(t)))
+		w.VarInt(int32(t))
+		if err := h.conn.SendWriter(protocol.SetCompressionID, w); err != nil {
+			return err
+		}
+		h.conn.EnableCompression(int32(t))
+	}
+
+	return h.sendLoginSuccess()
+}
+
+// sendLoginSuccess writes the Login Success packet. For protocol 775 the body
+// is: UUID, Username, then a VarInt-prefixed array of profile properties (none
+// in offline mode).
+func (h *handler) sendLoginSuccess() error {
+	p := h.conn.Profile
+	w := protocol.NewWriter(16 + len(p.Name) + 2)
+	w.UUID(p.UUID)
+	w.String(p.Name)
+	w.VarInt(0) // property count
+	return h.conn.SendWriter(protocol.LoginSuccessID, w)
+}
+
+// uuidString formats a UUID as the canonical 8-4-4-4-12 hex string.
+func uuidString(u [16]byte) string {
+	const hexdigits = "0123456789abcdef"
+	var b [36]byte
+	j := 0
+	for i := 0; i < 16; i++ {
+		if i == 4 || i == 6 || i == 8 || i == 10 {
+			b[j] = '-'
+			j++
+		}
+		b[j] = hexdigits[u[i]>>4]
+		b[j+1] = hexdigits[u[i]&0x0f]
+		j += 2
+	}
+	return string(b[:j])
+}