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/vanilla_parity_test.go

@@ -0,0 +1,45 @@
+package world
+
+import (
+	"encoding/json"
+	"math"
+	"os"
+	"strings"
+	"strconv"
+	"testing"
+)
+
+// TestVanillaParity compares our generated surface heights against heights
+// captured from the official server (seed 12345, normal terrain). Requires
+// /tmp/vanilla_ground.json from the capture step.
+func TestVanillaParity(t *testing.T) {
+	raw, err := os.ReadFile("/tmp/vanilla_ground.json")
+	if err != nil {
+		t.Skip("no vanilla capture")
+	}
+	var van map[string][]int
+	json.Unmarshal(raw, &van)
+
+	gen := NewVanillaGenerator(12345)
+	var total, exact, within1, within3 int
+	var maxDiff int
+	for key, vh := range van {
+		parts := strings.Split(key, ",")
+		cx, _ := strconv.Atoi(parts[0])
+		cz, _ := strconv.Atoi(parts[1])
+		ch := gen(int32(cx), int32(cz))
+		oh := ch.columnHeights()
+		for idx := 0; idx < 256; idx++ {
+			ourY := int(oh[idx]) - 65
+			d := int(math.Abs(float64(ourY - vh[idx])))
+			total++
+			if d == 0 { exact++ }
+			if d <= 1 { within1++ }
+			if d <= 3 { within3++ }
+			if d > maxDiff { maxDiff = d }
+		}
+	}
+	pct := func(n int) float64 { return 100 * float64(n) / float64(total) }
+	t.Logf("columns=%d exact=%.1f%% within1=%.1f%% within3=%.1f%% maxDiff=%d",
+		total, pct(exact), pct(within1), pct(within3), maxDiff)
+}