fix(lab): AR Passthrough black screen — transparent canvas + immersive-ar detection + dynamic AR background

- gl.alpha:true + setClearColor(0,0) no WebGL renderer
- ARBackground detecta environmentBlendMode='additive' e vira transparente
- Tenta navigator.xr.requestSession('immersive-ar') antes do fallback immersive-vr
- local-floor required + hand-tracking/plane-detection/hit-test optional

src/lab/QuestCubeLab.tsx

@@ -316,10 +316,11 @@ const PhysicsScene = ({
 // ===================================================================
 const SceneContent = ({ locked, resetSignal }: { locked: boolean; resetSignal: number }) => (
   <>
-    <color attach="background" args={["#0a0a0a"]} />
-    <fog attach="fog" args={["#0a0a0a", 5, 18]} />
-    <PerspectiveCamera makeDefault position={[0, 1.5, 3]} fov={50} />
-    <ambientLight intensity={0.4} />
+    {/* Em modo AR Passthrough, NÃO pinta cor de fundo — ela veda o passthrough */}
+    {/* Em modo desktop (fora do XR), pinta fundo escuro pra não ficar transparente estranho */}
+    <ARBackground />
+
+    <ambientLight intensity={0.6} />
     <directionalLight
       position={[5, 5, 5]}
       intensity={1.2}
@@ -327,7 +328,7 @@ const SceneContent = ({ locked, resetSignal }: { locked: boolean; resetSignal: n
       shadow-mapSize={[1024, 1024]}
     />
 
-    {/* Grid estilo HLA */}
+    {/* Grid estilo HLA — funciona em ambos os modos */}
     <Grid
       position={[0, 0.001, 0]}
       args={[20, 20]}
@@ -359,6 +360,21 @@ const SceneContent = ({ locked, resetSignal }: { locked: boolean; resetSignal: n
   </>
 );
 
+// ===================================================================
+// FUNDO DINÂMICO — VR imersivo fica preto; AR Passthrough fica transparente
+// ===================================================================
+const ARBackground = () => {
+  const session = useXR((s) => s.session);
+  const isAR = session && "environmentBlendMode" in session && (session as any).environmentBlendMode === "additive";
+  if (isAR) return null; // AR = transparente (mostra passthrough do Quest 3)
+  return (
+    <>
+      <color attach="background" args={["#0a0a0a"]} />
+      <fog attach="fog" args={["#0a0a0a", 5, 18]} />
+    </>
+  );
+};
+
 // ===================================================================
 // JOYSTICK HANDLER — gamepad pro controle fora do XR
 // ===================================================================
@@ -410,7 +426,39 @@ export default function QuestCubeLab() {
 
   useEffect(() => {
     if (autoEnter) {
-      store.enterXR?.().catch(() => {});
+      // Meta Quest 3 suporta immersive-ar (Passthrough AR). Tenta primeiro.
+      // Fallback pra immersive-vr se AR não disponível (óculos sem passthrough).
+      (async () => {
+        try {
+          if ('xr' in navigator) {
+            const xr = (navigator as any).xr;
+            const supported: boolean = await xr.isSessionSupported('immersive-ar').catch(() => false);
+            if (supported) {
+              const sessionInit: XRSessionInit = {
+                requiredFeatures: ['local-floor'],
+                optionalFeatures: ['hand-tracking', 'plane-detection', 'hit-test'],
+              };
+              await xr.requestSession('immersive-ar', sessionInit);
+              return;
+            }
+          }
+          // Fallback: usa o XR controller padrão do store
+          const ctrl: any = store as any;
+          const enterFn = ctrl.enterAR || ctrl.enterVR || ctrl.enterXR;
+          try {
+            await enterFn.call(ctrl, 'immersive-vr' as any);
+          } catch {
+            // ignore
+          }
+        } catch {
+          // qualquer erro, tenta o enter padrão
+          try {
+            await Promise.resolve(store.enterXR?.());
+          } catch {
+            // ignore
+          }
+        }
+      })();
     }
   }, [autoEnter]);
 
@@ -473,9 +521,17 @@ export default function QuestCubeLab() {
         Rapier Physics ativo • Half-Life: Alyx-like
       </div>
 
-      {/* Canvas com XR */}
+      {/* Canvas com XR — gl.alpha:true permite passthrough por trás */}
       <XR store={store}>
-        <Canvas shadows>
+        <Canvas
+          shadows
+          gl={{ alpha: true, antialias: true, powerPreference: "high-performance" }}
+          camera={{ position: [0, 1.5, 3], fov: 50 }}
+          onCreated={({ gl }) => {
+            gl.setClearColor(0x000000, 0); // transparente
+            gl.xr.enabled = true;
+          }}
+        >
           <SceneContent locked={locked} resetSignal={resetSignal} />
         </Canvas>
       </XR>