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/server.py

@@ -217,6 +217,7 @@ class MatchResp(BaseModel):
     find_time: float
     num_variants: int
     annotated_id: str
+    diag: dict | None = None  # CC: diagnostica pipeline (drop reasons)
 
 
 class TuneParams(BaseModel):
@@ -521,6 +522,7 @@ def match(p: MatchParams):
         ) for m_ in matches],
         train_time=t_train, find_time=t_find,
         num_variants=n, annotated_id=ann_id,
+        diag=m.get_last_diag() if hasattr(m, "get_last_diag") else None,
     )
 
 
@@ -596,6 +598,7 @@ def match_simple(p: SimpleMatchParams):
         ) for mt in matches],
         train_time=t_train, find_time=t_find,
         num_variants=n, annotated_id=ann_id,
+        diag=m.get_last_diag() if hasattr(m, "get_last_diag") else None,
     )
 
 
@@ -769,6 +772,7 @@ def match_recipe(p: RecipeMatchParams):
         ) for mt in matches],
         train_time=0.0, find_time=t_find,
         num_variants=len(m.variants), annotated_id=ann_id,
+        diag=m.get_last_diag() if hasattr(m, "get_last_diag") else None,
     )