refactor(agent): DRY pass on the GPU agent worker package

Consolidate genuine duplication in agent/fc_agent into single-source
helpers (behavior-preserving; DRY Pass process #594):

worker.py
- _fail(jid, image_id, exc, verb) — 4 terminal "fail this job" blocks
  (downloader HTTP-fault + decode, consumer non-transient + generic).
- _release(job_ids) (was _release_owned) — the one lease hand-back path;
  6 inline release([jid])+unhold sites now route through it.
- _stopped(stop_evt) + _abort_if_stopped(jid, stop_evt) — 4 stop-check
  -and-release blocks and every bare stop-check.
- _timed(stage) contextmanager — ~8 monotonic()/_record() timing pairs;
  records only on clean exit, matching the old skip-on-raise behavior.
- _ewma(prev, x, alpha) module fn — 3 EWMA updates in the autoscaler.

client.py
- _submit(path, payload) — submit / submit_embedding (retrying session).
- _post_quiet(path, payload) — heartbeat / fail / release fire-and-forget.

detectors.py
- Proposers._top(detector, image, cap) — merges components() and panels().

config.py
- _bool_env(name, default) — auto_start / auto_scale env parsing.

Left alone (recorded): the xyxy→norm-xywh conversion duplicated across
models.py/detectors.py (2 copies, independent wrapper modules — sharing
would couple them), and the _ensure_embedder/_ensure_proposers pair (same
lock shape, different concepts).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

agent/fc_agent/detectors.py

@@ -202,14 +202,17 @@ class Proposers:
             boxes += self._person.detect(image)
         return nms_merge(boxes)[: self.cfg.max_figures]   # nms_merge is score-desc
 
-    def components(self, image):
-        if self._anatomy is None:
+    @staticmethod
+    def _top(detector, image, cap: int):
+        """Top-`cap` detections by score from an optional proposer (None → the
+        proposer is off → []). Shared by the anatomy + panel proposers, which
+        differ only in which detector and which cap."""
+        if detector is None:
             return []
-        items = sorted(self._anatomy.detect(image), key=lambda b: b[1], reverse=True)
-        return items[: self.cfg.max_components]
+        return sorted(detector.detect(image), key=lambda b: b[1], reverse=True)[:cap]
+
+    def components(self, image):
+        return self._top(self._anatomy, image, self.cfg.max_components)
 
     def panels(self, image):
-        if self._panel is None:
-            return []
-        items = sorted(self._panel.detect(image), key=lambda b: b[1], reverse=True)
-        return items[: self.cfg.max_panels]
+        return self._top(self._panel, image, self.cfg.max_panels)