knn scatter: exclusive demo (not additive)

Same pattern as models / perf / live: cachedReal accumulates real
embedding events at all times; demoActive flag gates which source
renders.
- demo on  → only synthetic clusters
- demo off → only real embeddings (replayed from cachedReal)

Cache cap 5000 points to bound memory across long sessions.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

training/dashboard/static/dashboard.js

@@ -2102,8 +2102,29 @@ def train_nn(*, model, X_train, y_train, X_val, y_val,
       rebuildLegend();
     }
 
-    on('demo_start', loadSynthetic);
-    on('demo_stop',  () => { points.length = 0; resetStats(); rebuildLegend(); });
+    // Exclusive: demo on → only synthetic clusters; demo off → only
+    // real embedding events. cachedReal accumulates real points
+    // even while demo is on, so demo_stop restores them.
+    let demoActive = false;
+    const cachedReal = [];
+    const REAL_CACHE_CAP = 5000;
+    function repaintFromReal() {
+      points.length = 0;
+      resetStats();
+      for (const pt of cachedReal) {
+        points.push(pt);
+        addStat(pt);
+      }
+      rebuildLegend();
+    }
+    on('demo_start', () => {
+      demoActive = true;
+      loadSynthetic();
+    });
+    on('demo_stop', () => {
+      demoActive = false;
+      repaintFromReal();
+    });
     on('embedding', m => {
       if (typeof m.x !== 'number' || typeof m.y !== 'number') return;
       const pt = {
@@ -2113,9 +2134,13 @@ def train_nn(*, model, X_train, y_train, X_val, y_val,
         predicted: m.predicted,
         cluster: typeof m.cluster === 'number' ? m.cluster : undefined,
       };
-      points.push(pt);
-      addStat(pt);
-      rebuildLegend();
+      cachedReal.push(pt);
+      if (cachedReal.length > REAL_CACHE_CAP) cachedReal.shift();
+      if (!demoActive) {
+        points.push(pt);
+        addStat(pt);
+        rebuildLegend();
+      }
     });
 
     rebuildLegend();

training/dashboard/static/index.html

@@ -1314,6 +1314,6 @@
     </article>
   </div>
 
-  <script src="/static/dashboard.js?v=4346accf"></script>
+  <script src="/static/dashboard.js?v=33d3c0e6"></script>
 </body>
 </html>