feat(migration): V7 核算台账/月结/调整单四表与核算纯函数

- V7 四表：performance_ledgers（append-only 版本行，uk(receipt_id,calc_version)
  库级防重兜底）/monthly_settlements（月份 uk，通过即锁定终态）/
  settlement_items（按人固化可负工资，uk 三列）/adjustment_orders（TZ 单号）
- registry +1：approval.adjustment.enabled（19 项，BOOL 首组扩至三开关）
- formula 补核算纯函数：clamp_weight_r（从预览内联提取共用）/monthly_salary
  三口径/probation_annual_note/manager_team_share（#12 暂口径：基数不含
  经理个人户业绩，聚合集中一处答复后只改）
- 迁移 downgrade 只保留 drop_table+alter 回滚+种子行反向 DELETE；
  performance_ledgers 表名复数（test_models 红线断言）
- 断言同步：EXPECTED_TABLES 四新表/registry 19/BOOL 三开关/conftest
  清理四新表与 ParamValue 边界 22
- schema_sync+全量单测 460 passed

Co-Authored-By: Claude Code <noreply@anthropic.com>

backend/app/services/param_service.py

@@ -614,11 +614,8 @@ def preview(session: Session, payload: ParamPreviewIn) -> ParamPreviewOut:
 
     start_ratio = take("payment.start.ratio")
     start_ok = formula.start_payment_ok(received, revenue, start_ratio)
-    # R 按「回款占销售额百分比向下取整」口径封顶封底：received 非负时 int()
-    # 即向下取整；min/max 同时是红字冲销（负数回款）接入时的方向防线
-    weight_r = (
-        max(0, min(100, int(Fraction(received) / Fraction(revenue) * 100))) if revenue > 0 else 0
-    )
+    # R 钳制已提取为纯函数 clamp_weight_r（④ 核算与预览共用同一实现）
+    weight_r = formula.clamp_weight_r(received, revenue)
     weight = formula.payment_weight(weight_r)
     base_amount = after_barter if barter_applied else performance
     payable = to_decimal(Fraction(base_amount) * weight)