feat: per-class max box fraction (REQ-188)

- labeling/preview/autolabel: max_box_frac + per-class max_box_fracs
  ceiling filter (0=none, 1=off) before NMS, mirrors min_box_frac
- exemplar review-assist: ceiling before max_detections truncation;
  ExemplarLabelRequest + filter panel 'Max box size' slider
- ClassParamsTable: MaxBox column; copy line now REQ-186 amended
  order: class <name> conf <v> iou <v> minbox <v> maxbox <v> container
- docs: design/ui-spec/tasks updated (incl. stale 4-slider narrative)

backend/preview.py

@@ -120,6 +120,9 @@ def preview_frame(
                     mb = cp.get("min_box_frac")
                     if mb and mb > 0 and (xyxyn[2]-xyxyn[0])*(xyxyn[3]-xyxyn[1]) < mb:
                         continue
+                    xb = cp.get("max_box_frac")
+                    if xb and xb < 1 and (xyxyn[2]-xyxyn[0])*(xyxyn[3]-xyxyn[1]) > xb:
+                        continue
 
                 all_raw_detections.append(labeling.Detection(
                     class_id=target_class_id,
@@ -143,7 +146,7 @@ def preview_frame(
                  if c["name"].strip().lower() == wanted),
                 -1,
             )
-        thr_list = iou_list = mb_list = None
+        thr_list = iou_list = mb_list = mx_list = None
         if per_class:
             names = [c["name"].strip().lower() for c in sam3_target_classes]
             if any("threshold" in v for v in per_class.values()):
@@ -152,6 +155,8 @@ def preview_frame(
                 iou_list = [per_class.get(n, {}).get("iou_threshold", iou_threshold) for n in names]
             if any("min_box_frac" in v for v in per_class.values()):
                 mb_list = [per_class.get(n, {}).get("min_box_frac", min_box_frac) for n in names]
+            if any("max_box_frac" in v for v in per_class.values()):
+                mx_list = [per_class.get(n, {}).get("max_box_frac", 1.0) for n in names]
         res = labeling.label_image(
             frame_file, frame["filename"], prompts, threshold,
             iou_threshold=iou_threshold, min_box_frac=min_box_frac,
@@ -159,6 +164,7 @@ def preview_frame(
             thresholds=thr_list,
             iou_by_class=dict(enumerate(iou_list)) if iou_list else None,
             min_box_fracs=mb_list,
+            max_box_fracs=mx_list,
             container_ids=prompt_container_ids,
         )
         if not res.error and res.detections: