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/app.js

@@ -336,6 +336,7 @@ async function doMatchRecipe() {
   document.getElementById("t-find").textContent = `${data.find_time.toFixed(2)}s`;
   document.getElementById("t-var").textContent = data.num_variants;
   document.getElementById("t-match").textContent = data.matches.length;
+  renderDiag(data.diag, data.matches.length);
   setStatus(`${data.matches.length} match trovati (ricetta ${state.active_recipe})`);
 }
 
@@ -409,6 +410,7 @@ async function doMatch() {
   document.getElementById("t-find").textContent = `${data.find_time.toFixed(2)}s`;
   document.getElementById("t-var").textContent = data.num_variants;
   document.getElementById("t-match").textContent = data.matches.length;
+  renderDiag(data.diag, data.matches.length);
   setStatus(`${data.matches.length} match trovati${hasAdv ? " (avanzato)" : ""}`);
 }
 
@@ -436,6 +438,61 @@ function setStatus(s) {
 }
 
 // ---------- Init ----------
+// ---------- CC: Diagnostica match ----------
+function renderDiag(diag, n_matches) {
+  const el = document.getElementById("diag-content");
+  if (!diag) {
+    el.innerHTML = '<em style="color:#888">Diagnostica non disponibile</em>';
+    return;
+  }
+  const dropTotal = (diag.drop_ncc_low || 0) + (diag.drop_min_score_post_avg || 0)
+    + (diag.drop_recall_low || 0) + (diag.drop_bbox_out_of_scene || 0)
+    + (diag.drop_nms_iou || 0);
+  // Hint contestuali se 0 match
+  let hint = "";
+  if (n_matches === 0) {
+    if (diag.n_after_pre_nms === 0) {
+      hint = `<div style="color:#f88; margin-top:6px">⚠ Nessun candidato sopra soglia.
+        Prova: ↓ <b>min_score</b> o ↓ <b>top_thresh</b> (currently ${diag.top_thresh_used.toFixed(2)})</div>`;
+    } else if (diag.drop_ncc_low > 0 && dropTotal === diag.drop_ncc_low) {
+      hint = `<div style="color:#f88; margin-top:6px">⚠ ${diag.drop_ncc_low} candidati droppati da NCC.
+        Prova: ↓ <b>verify_threshold</b> (filtro_fp più leggero)</div>`;
+    } else if (diag.drop_min_score_post_avg > 0) {
+      hint = `<div style="color:#f88; margin-top:6px">⚠ ${diag.drop_min_score_post_avg} match sotto min_score post-NCC.
+        Prova: ↓ <b>min_score</b></div>`;
+    } else if (diag.drop_recall_low > 0) {
+      hint = `<div style="color:#f88; margin-top:6px">⚠ ${diag.drop_recall_low} match con recall < ${diag.min_recall_used}.
+        Prova: ↓ <b>min_recall</b></div>`;
+    } else if (diag.drop_bbox_out_of_scene > 0) {
+      hint = `<div style="color:#f88; margin-top:6px">⚠ ${diag.drop_bbox_out_of_scene} match con bbox fuori scena.
+        Centro derivato male: aumenta <b>min_score</b> o restringi <b>search_roi</b></div>`;
+    }
+  }
+  const flags = [];
+  if (diag.use_polarity) flags.push("polarity");
+  if (diag.use_soft_score) flags.push("soft");
+  if (diag.subpixel_lm) flags.push("subpix-LM");
+  el.innerHTML = `
+    <div><b>Pipeline pruning:</b></div>
+    <div>varianti: ${diag.n_variants_total} → top_eval=${diag.n_variants_top_evaluated}
+       → top_pass=${diag.n_variants_top_passed} → full_eval=${diag.n_variants_full_evaluated}</div>
+    <div><b>Candidati:</b> raw=${diag.n_raw_candidates}
+       → pre_nms=${diag.n_after_pre_nms} → final=${diag.n_final}</div>
+    <div><b>Drop reasons:</b> NCC=${diag.drop_ncc_low}, score=${diag.drop_min_score_post_avg},
+       recall=${diag.drop_recall_low}, bbox=${diag.drop_bbox_out_of_scene}, NMS=${diag.drop_nms_iou}</div>
+    <div><b>Soglie:</b> top=${diag.top_thresh_used.toFixed(2)},
+       min_score=${diag.min_score_used.toFixed(2)},
+       NCC=${diag.verify_threshold_used.toFixed(2)},
+       recall=${diag.min_recall_used.toFixed(2)}</div>
+    ${flags.length ? `<div><b>Flag attivi:</b> ${flags.join(", ")}</div>` : ""}
+    ${hint}
+  `;
+  // Auto-apri pannello se 0 match (segnala problema)
+  if (n_matches === 0) {
+    document.getElementById("diag-panel").open = true;
+  }
+}
+
 // ---------- Auto-tune (Halcon-style) ----------
 async function doAutoTune() {
   if (!state.model || !state.roi) {