Fail fast on missing terrain assets and recover empty renders

renderer/terrain-mesh.js

@@ -225,6 +225,11 @@ export async function buildTerrain(
     );
   }
   const modelMs = performance.now() - started - indexMs;
+  if (!materials.size || !entries.some(([, entry]) => entry.faces.length)) {
+    throw new Error(
+      `No terrain models could be rendered. Minecraft models/textures are missing or failed to load. Run npm run setup. Unsupported examples: ${[...unsupported].slice(0, 8).join(", ")}`,
+    );
+  }
   await progress(35, "Building cached block faces", true);
   const batches = new Map(),
     meshList = [],
@@ -299,6 +304,11 @@ export async function buildTerrain(
     );
   }
   window.terrainBounds = snapshot.bounds;
+  if (!meshList.length || !emittedFaces) {
+    throw new Error(
+      "Terrain renderer produced no visible geometry. The export cannot be accepted as a playable map.",
+    );
+  }
   window.terrainMeshes = meshList;
   const totalMs = performance.now() - started;
   return {