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_bench_test.go

@@ -0,0 +1,7 @@
+package world
+import "testing"
+func BenchmarkGenerateVanilla(b *testing.B){
+	g:=NewVanillaGenerator(12345)
+	b.ResetTimer(); b.ReportAllocs()
+	for i:=0;i<b.N;i++{ _=g(int32(i),0) }
+}