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/golden_test.go

@@ -0,0 +1,46 @@
+package world
+
+import (
+	"bytes"
+	"os"
+	"testing"
+
+	"regionio/internal/protocol"
+)
+
+// extractSectionData returns the section-data byte slice of a level_chunk body
+// (the bytes covered by the VarInt size that follows the heightmaps).
+func extractSectionData(t *testing.T, body []byte) []byte {
+	t.Helper()
+	r := protocol.NewReader(body[8:]) // skip chunk X,Z
+	count, _ := r.VarInt()
+	for i := int32(0); i < count; i++ {
+		r.VarInt()         // heightmap type
+		n, _ := r.VarInt() // long count
+		for j := int32(0); j < n; j++ {
+			r.Int64()
+		}
+	}
+	size, _ := r.VarInt()
+	consumed := len(body[8:]) - r.Remaining()
+	return body[8+consumed : 8+consumed+int(size)]
+}
+
+// TestGoldenAgainstVanilla asserts our flat-chunk section data is byte-for-byte
+// identical to a chunk captured from the official 26.1.2 server (same world
+// coordinate). This guards the paletted-container and heightmap encoding.
+// Light is intentionally not compared (we send full-bright, which differs).
+func TestGoldenAgainstVanilla(t *testing.T) {
+	vanilla, err := os.ReadFile("testdata/vanilla_flat_chunk.bin")
+	if err != nil {
+		t.Fatalf("read fixture: %v", err)
+	}
+
+	ours := GenerateFlat(0, -1).Encode() // fixture was captured at chunk (0, -1)
+	want := extractSectionData(t, vanilla)
+	got := extractSectionData(t, ours)
+
+	if !bytes.Equal(want, got) {
+		t.Fatalf("section data differs: vanilla=%d bytes, ours=%d bytes", len(want), len(got))
+	}
+}