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/terrain_debug_test.go

@@ -0,0 +1,30 @@
+package world
+
+import (
+	"fmt"
+	"testing"
+	"regionio/internal/worldgen"
+)
+
+func TestTerrainHeightProfile(t *testing.T) {
+	d := worldgen.SimpleTerrain(0)
+	minH, maxH := 1000, -1000
+	// surface height along z=8 for x in [-32,32]
+	line := ""
+	for x := -32; x <= 32; x += 4 {
+		top := MinY - 1
+		for y := MinY; y < MinY+WorldHeight; y++ {
+			if d.Compute(worldgen.FunctionContext{X: float64(x), Y: float64(y), Z: 8}) > 0 {
+				top = y
+			}
+		}
+		line += fmt.Sprintf("%d ", top)
+		if top < minH { minH = top }
+		if top > maxH { maxH = top }
+	}
+	t.Logf("surface heights (z=8): %s", line)
+	t.Logf("min=%d max=%d range=%d", minH, maxH, maxH-minH)
+	if minH < MinY || maxH > 120 {
+		t.Fatalf("implausible terrain heights")
+	}
+}