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/worldgen/loadtest_test.go

@@ -0,0 +1,27 @@
+package worldgen
+
+import (
+	"math"
+	"testing"
+)
+
+// TestLoadOverworldDensity loads the full overworld final_density tree and
+// checks it evaluates to finite values with the expected vertical sign trend
+// (solid deep down, air high up).
+func TestLoadOverworldDensity(t *testing.T) {
+	od, err := LoadOverworldFinalDensity(0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	deep := od.Final.Compute(FunctionContext{X: 0, Y: -40, Z: 0})
+	high := od.Final.Compute(FunctionContext{X: 0, Y: 200, Z: 0})
+	if math.IsNaN(deep) || math.IsNaN(high) {
+		t.Fatal("final_density produced NaN")
+	}
+	if !(deep > 0) {
+		t.Fatalf("expected solid (positive) deep underground, got %v", deep)
+	}
+	if !(high < 0) {
+		t.Fatalf("expected air (negative) high up, got %v", high)
+	}
+}