fix(工资核算): 排除起付比例中的历史发货

旧应收已经由期初余额承接，启用前发货再次进入分母，
会使新业务足额回款仍未达到起付门槛。

将销售额日期范围与 FIFO 的启用边界保持一致，
补充普通历史日期及二月末的两项回归用例。

backend/app/services/payroll_calculation.py

@@ -25,7 +25,7 @@ from app.services import audit_service, formula, payroll_months
 from app.services.audit_service import AuditRecord
 from app.services.param_service import resolve_snapshot
 from app.services.tplus.calculation_preview import _parameters
-from app.services.tplus.fifo import _beneficiary, amount_text, money, preview_group
+from app.services.tplus.fifo import START, _beneficiary, amount_text, money, preview_group
 from app.services.tplus.workbench import digest
 
 
@@ -308,11 +308,13 @@ def _calculate_month(session, business, record, month, reason=None):
             }
     if set(previous) - current.keys():
         raise BadRequestError("历史已核算事件发生变化，请核对更正或调整，不可直接覆盖")
+    # 启用前的发货由期初应收承接，不再进入新业务的起付比例分母。
     sales = sum(
         (
             money(s["amount"])
             for s in values["shipments"]
-            if s["date"] <= until.isoformat() and not s.get("performance_excluded")
+            if START.isoformat() <= s["date"] <= until.isoformat()
+            and not s.get("performance_excluded")
         ),
         Decimal(0),
     )