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/labeling.py

@@ -112,19 +112,23 @@ def label_image(
     threshold: float,
     iou_threshold: float = 0.8,
     min_box_frac: float = 0.0,
+    max_box_frac: float = 1.0,
     exemplar_index: int = -1,
     exemplars: Optional[List[dict]] = None,
     thresholds: Optional[List[float]] = None,
     iou_by_class: Optional[Dict[int, float]] = None,
     min_box_fracs: Optional[List[float]] = None,
+    max_box_fracs: Optional[List[float]] = None,
     container_ids: Optional[Set[int]] = None,
 ) -> ImageResult:
     """Detect every prompt in one image and return the surviving instances.
 
     When `exemplars` are given, the prompt at `exemplar_index` also carries them
     as drawn box exemplars (REQ-172); every other prompt runs on text alone.
-    `thresholds`, `iou_by_class` and `min_box_fracs` are per-prompt overrides
-    aligned with `prompts` (REQ-181); classes without one use the global values.
+    `thresholds`, `iou_by_class`, `min_box_fracs` and `max_box_fracs` are
+    per-prompt overrides aligned with `prompts` (REQ-181); classes without one
+    use the global values. `max_box_frac(s)` is the per-class size ceiling,
+    1.0 = off (REQ-188).
     `container_ids` are prompt indices marked container (REQ-184) — the caller
     maps them, because detections still carry prompt-index class ids here."""
     try:
@@ -160,6 +164,21 @@ def label_image(
             if (d.box[2] - d.box[0]) * (d.box[3] - d.box[1]) >= floor
         ]
 
+    if max_box_fracs is not None:
+        def _keep_max(det) -> bool:
+            frac = max_box_fracs[det.class_id] if det.class_id < len(max_box_fracs) else max_box_frac
+            if frac >= 1 or frac <= 0:
+                return True
+            ceiling = width * height * frac
+            return (det.box[2] - det.box[0]) * (det.box[3] - det.box[1]) <= ceiling
+        detections = [d for d in detections if _keep_max(d)]
+    elif 0 < max_box_frac < 1:
+        ceiling = width * height * max_box_frac
+        detections = [
+            d for d in detections
+            if (d.box[2] - d.box[0]) * (d.box[3] - d.box[1]) <= ceiling
+        ]
+
     return ImageResult(image_path, rel_path, width, height,
                        deduplicate(detections, iou_threshold, iou_by_class=iou_by_class,
                                    container_ids=container_ids))