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/config.py

@@ -34,6 +34,8 @@ class Config:
     panel_conf: float
     max_components: int   # cap anatomy component crops per frame
     max_panels: int       # cap panel crops per frame
+    max_figures: int      # cap figure boxes per frame (each = a CCIP call + crop)
+    max_regions: int      # hard cap on total regions per JOB (submit-size backstop)
 
     @classmethod
     def from_env(cls) -> Config:
@@ -58,4 +60,6 @@ class Config:
             panel_conf=float(os.environ.get("PANEL_CONF", "0.30")),
             max_components=int(os.environ.get("MAX_COMPONENTS", "8")),
             max_panels=int(os.environ.get("MAX_PANELS", "8")),
+            max_figures=int(os.environ.get("MAX_FIGURES", "8")),
+            max_regions=int(os.environ.get("MAX_REGIONS", "128")),
         )