feat(playground): wire WasmIML methods into debug probe

Add evalLoss, inferBatch, and getLayerStats to the window.__nisps
debug probe so Playwright tests and dev console can access the new
WasmIML capabilities. InputHeatmap (Phase 3) is not yet wired into
a-app.js, so batch inference heatmap integration is deferred.

playground/js/a-app.js

@@ -914,6 +914,9 @@ async function init() {
         updateStatus();
       },
       saveState: () => saveState(),
+      evalLoss:     () => iml.evalLoss(),
+      inferBatch:   (points) => iml.inferBatch(points),
+      getLayerStats:() => iml.getLayerStats(),
     };
   }