integrate: restore CostStore import dropped with #241 dual_column_view line

The #241 import resolution deleted the whole 'from cloud.cost import
CostStore, dual_column_view' line to drop the unused alias, which
also removed the CostStore import that _quota_snapshot needs; every
task creation then failed closed with 503 quota_check_unavailable
(4 EndpointChainTestCase failures in test_audit_wiring). Restored
as a plain CostStore import; dual_column_view stays available in
cloud/cost.py for #241's own tests.

cloud/server.py

@@ -75,9 +75,9 @@ from cloud.cost import CostStore, unpriced_token_cap
 from cloud.models import evaluate_capability as _evaluate_capability
 from cloud.models import load_routing_policy as _load_routing_policy
 from cloud.routing_shadow import evaluate_shadow as _evaluate_shadow
-# 演练注：本提交原 diff 曾删 CostStore 导入（其树内仍有 5 处
-# CostStore 调用，属源线中间态潜 bug，后续源提交又拆行补回）；
-# 此处取并集保留导入，行为与源线终态一致。
+# 演练注：eeb1011 曾删 CostStore 导入（其树内仍有调用，属源线
+# 中间态潜 bug）；本提交即源线内补回该导入的提交，因上一冲突
+# 解法已提前保留导入，此处其意图已满足，不重复加行。
 from cloud.gates import merge_state_decision as _merge_state_decision
 from cloud.gates import resolve_inside as _gate_resolve_inside
 from cloud.ledger import export_chain as _ledger_export