feat(web): pannello diagnostica match (CC) con hint contestuali

MatchResp ora include diag dict (CC feature). UI rendering:

- Nuovo pannello pieghevole "🔍 Diagnostica" sotto i tempi
- Per ogni match mostra:
  * pipeline pruning (vars total → top_eval → top_pass → full_eval)
  * candidati (raw → pre_nms → final)
  * drop reasons (NCC, score, recall, bbox, NMS) con counter
  * soglie effettive applicate
  * flag attivi (polarity, soft, subpix-LM)

- Quando 0 match → pannello si apre automaticamente + mostra hint
  contestuale specifico:
  * "0 candidati top" → suggerisce ↓ min_score / top_thresh
  * "tutti dropped da NCC" → ↓ verify_threshold (filtro_fp)
  * "score post-NCC sotto" → ↓ min_score
  * "recall basso" → ↓ min_recall
  * "bbox out-of-scene" → check pose / search_roi

Risolve il pattern "0 match perche'?" con guida actionable invece
del black-box. Tutti e 3 endpoint match (/match, /match_simple,
/match_recipe) propagano il diag.

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

pm2d/web/static/index.html

@@ -214,6 +214,16 @@
     <div class="kv"><span>find:</span><span id="t-find">-</span></div>
     <div class="kv"><span>varianti:</span><span id="t-var">-</span></div>
     <div class="kv"><span>match:</span><span id="t-match">-</span></div>
+
+    <details id="diag-panel" style="margin-top:10px">
+      <summary>🔍 Diagnostica (CC)</summary>
+      <div id="diag-content" style="font-family:monospace; font-size:11px;
+                                     background:#1a1a1a; padding:8px;
+                                     border-radius:3px; margin-top:6px;
+                                     line-height:1.5">
+        <em style="color:#888">Esegui un MATCH per vedere la diagnostica</em>
+      </div>
+    </details>
   </section>
 </main>