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/density_bench_test.go

@@ -0,0 +1,7 @@
+package worldgen
+import "testing"
+func BenchmarkFinalDensity(b *testing.B) {
+	od,_ := LoadOverworldFinalDensity(0)
+	b.ReportAllocs()
+	for i:=0;i<b.N;i++ { _ = od.Final.Compute(FunctionContext{X:float64(i&255),Y:64,Z:float64(i>>8)}) }
+}