feat: routing shadow evaluation v0b (read-only, never auto-activates)

New pure module cloud/routing_shadow.py: parse worker_routing
audit details (old format without suggested model parses with
suggested unknown, never guessed), evaluate per-tier routed
samples with one-task-one-sample dedup, explicit tasks counted
separately, status under the chosen model, token/USD with
unpriced tokens kept apart, and a min-20 gate per tier. The
report explicitly marks the counterfactual not_measured:
shadow suggested models never ran, so no suggested-vs-chosen
cost/quality comparison is possible. Worker audit detail now
includes suggested/chosen for future rows. GET /routing-shadow
serves the report (project read permission, truncated flag
over 10000 audit rows, cost table ensured so a fresh DB does
not 500).

Tests: test_routing_shadow.py (5 tests).

cloud/worker.py

@@ -5920,10 +5920,17 @@ class Worker:
             # 不需要再用一条事件重复说明。
             if decision.source in ("policy", "shadow"):
                 record_routing({"id": task_id, "events": events}, decision)
+                # 影子评估（v0b）需要建议模型：旧格式只有
+                # tier/source/applied，评估时建议模型只能记
+                # unknown；新格式补 suggested/chosen，旧解析
+                # 器仍能读（见 cloud/routing_shadow.py）。
                 self._audit(task["project_id"], "worker_routing",
-                            "%s tier=%s source=%s applied=%s" % (
+                            "%s tier=%s source=%s applied=%s"
+                            " suggested=%s chosen=%s" % (
                                 task_id, decision.tier, decision.source,
-                                decision.applied))
+                                decision.applied,
+                                decision.suggested_model or "-",
+                                decision.chosen_model or "-"))
             task["model"] = decision.chosen_model or self.model
         # agent 指令通道（v2）：local/docker 由 sandbox 的
         # --developer-instructions 派发，prompt 保持干净；通用