fix(agent): cap figures + global region cap + reset active on stop

Three safety/robustness fixes from the operator's run logs:

- Cap figures per frame (MAX_FIGURES, default 8) like components/panels already
  are. Uncapped, a huge/busy image yielded hundreds of figure boxes → hundreds
  of per-figure CCIP calls + crops → a 38s job AND a submit too big to accept
  (image 81602 looped on 413). This is the acute fix.
- Global per-JOB backstop (MAX_REGIONS, default 128): if total regions still
  exceed the cap (long video), keep the highest-scoring and log the drop, so a
  submit body can never blow past curator's limit.
- Stale "active" meter: stop() now resets _active to 0 (no slots remain, so the
  meter must read 0 at once), and _bump clamps at 0 so a slot finishing after the
  reset can't drive it negative.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

agent/fc_agent/detectors.py

@@ -144,11 +144,14 @@ class Proposers:
 
     def figures(self, image, base_boxes):
         """Merge imgutils person boxes (base_boxes: [(bbox, score)]) with the
-        general COCO person detector → NMS'd figure boxes [(bbox, score, label)]."""
+        general COCO person detector → NMS'd figure boxes [(bbox, score, label)],
+        capped to the highest-scoring max_figures. Uncapped, a busy/huge image
+        (many characters) yields hundreds of boxes → hundreds of per-figure CCIP
+        calls + crops → a 30s+ job and an oversized submit (operator-flagged)."""
         boxes = [(bb, sc if sc is not None else 1.0, "person") for bb, sc in base_boxes]
         if self._person is not None:
             boxes += self._person.detect(image)
-        return nms_merge(boxes)
+        return nms_merge(boxes)[: self.cfg.max_figures]   # nms_merge is score-desc
 
     def components(self, image):
         if self._anatomy is None: