fix: 4 critical optimizer bugs

- NaN composite score: dropna().mean() on empty series returns NaN,
  nan is truthy so (nan or 0)=nan. Fixed: use None when no MFE data
  so scorer uses 0.5 fallback instead of propagating NaN.

- Analysis stopping: gate WFV was importing from main.execute_run
  (old CLI code that uses broken runner). Fixed: gate.run_walk_forward
  now accepts an executor callable from optimizer_loop.

- Back to Dashboard opened blank tab: Reports button used
  window.open(_blank). Fixed: same-tab navigation + history.back().

- NaN in reports card: downstream of composite_score NaN above.

ui/templates/index.html

@@ -39,7 +39,7 @@
       </div>
     </div>
     <div class="header-right">
-      <button class="btn btn-secondary" id="btn-reports" onclick="window.open('/reports','_blank')">
+      <button class="btn btn-secondary" id="btn-reports" onclick="window.location.href='/reports'">
         📁 Reports
       </button>
       <button class="btn btn-pause hidden" id="btn-pause" onclick="pauseOptimizer()">

ui/templates/reports_index.html

@@ -38,7 +38,7 @@
       <h1>📁 Optimization Reports</h1>
       <div class="sub">{{ runs|length }} run(s) recorded · Click any card to open the full report</div>
     </div>
-    <a class="back-link" href="/">← Back to Dashboard</a>
+    <a class="back-link" href="/" onclick="if(document.referrer){{history.back();return false;}}">← Back to Dashboard</a>
   </header>
 
   {% if not runs %}