viewer: log GLB load errors + surface a friendly message instead of silently empty stage

index.html

@@ -570,7 +570,12 @@ async function loadItem(cls, item) {
   if (_sl) { _slt.textContent = `loading ${item.id}…`; _sl.classList.add("active"); }
   let gltf;
   try { gltf = await loader.loadAsync(dsUrl(cls.dataset, item.glb)); }
-  finally { if (_sl) _sl.classList.remove("active"); }
+  catch (err) {
+    console.error(`GLB load failed for ${item.id}:`, err);
+    if (_sl) { _slt.textContent = `couldn\u2019t load ${item.id}`; }
+    return;
+  }
+  finally { if (_sl && !_sl.classList.contains("error")) _sl.classList.remove("active"); }
   if (gen !== loadGen) return;   // user already switched away — drop this load
   model = new THREE.Group();
   // Shipped GLBs are Y-up (Blender glTF export); catalog pivots/axes are also