fix: MCQ double-count — remove duplicate onclick handler in showMCQ

wireButtons() already registers addEventListener on MCQ buttons (once,
guarded by _pp). showMCQ() was also setting btn.onclick, causing
handleMCQAnswer to fire twice per click and push two history entries.
Remove the onclick assignment and add mcqAnswered guard for safety.

app.py

@@ -558,7 +558,7 @@ BRIDGE_JS = """() => {
 
   // ── MCQ display ────────────────────────────────────────────────────────────
   function showMCQ(data) {
-    S.mcqData = data; S.waitingMCQ = false;
+    S.mcqData = data; S.waitingMCQ = false; S.mcqAnswered = false;
     const qt = $('q-text'), ql = $('q-loading'), nq = $('new-q-btn');
     const es = $('empty-state'), qa = $('question-area'), eb = $('end-btn');
     if (es) es.classList.add('hidden');
@@ -573,7 +573,7 @@ BRIDGE_JS = """() => {
       if (txt) txt.textContent = (data.choices && data.choices[l]) || '';
       if (btn) {
         btn.className = 'mcq-btn'; btn.disabled = false;
-        btn.onclick = ((letter) => () => handleMCQAnswer(letter))(l);
+        btn.onclick = null;
       }
     }
     $('mcq-options')?.classList.remove('hidden');
@@ -585,7 +585,8 @@ BRIDGE_JS = """() => {
   // ── MCQ answer handler (client-side — no LLM call needed) ─────────────────
   function handleMCQAnswer(letter) {
     const data = S.mcqData;
-    if (!data || !data.correct || S.waitingMCQ) return;
+    if (!data || !data.correct || S.waitingMCQ || S.mcqAnswered) return;
+    S.mcqAnswered = true;
     const correct = data.correct;
     const isCorrect = letter === correct;
     for (const l of ['A','B','C','D']) {