feat(system): 计提参数按业务日期时点化取值服务

- param_repository.resolve_values：窗口函数一条 SQL 取全量快照（同日多次变更取 effective_date DESC, id DESC 首条）
- param_service.ParamSnapshot：冻结快照 + 覆盖链（角色 > 产品类别 > 默认行）+ 注册表默认值兜底；fraction/integer/boolean/text 类型化取值
- 12 项单元用例：覆盖链优先级、兜底、同日取最新、冻结不可变

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

backend/tests/unit/test_param_snapshot.py

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+"""参数快照（时点化取值）测试。
+
+守的是：覆盖链与兜底 —— 一旦取错维度（如企业户取到市政阈值行）
+或漏兜底，核算会用错口径或直接崩溃；同日多次变更必须取最新留痕。
+仓储 SQL 的排序语义（同日取 id 最大）在集成测试中覆盖。
+"""
+
+from dataclasses import FrozenInstanceError
+from datetime import date
+from fractions import Fraction
+
+import pytest
+
+from app.services.param_service import ParamSnapshot
+
+
+def _snap(rows: list[tuple[str, str, str]]) -> ParamSnapshot:
+    """构造快照。rows 须模拟仓储输出：同一 (code, dim) 的最新值在前。"""
+    return ParamSnapshot.from_rows(date(2026, 10, 1), rows)
+
+
+class TestCoverChain:
+    """覆盖链：角色维度 > 产品类别维度 > 默认行（''）。"""
+
+    def test_产品类别维度优先于默认行(self) -> None:
+        snap = _snap([("enterprise.rate", "", "3"), ("enterprise.rate", "municipal_water", "2")])
+        assert snap.get("enterprise.rate", product_category="municipal_water") == "2"
+
+    def test_无匹配维度时落默认行(self) -> None:
+        snap = _snap([("enterprise.rate", "", "3"), ("enterprise.rate", "municipal_water", "2")])
+        assert snap.get("enterprise.rate", product_category="rail_transit") == "3"
+
+    def test_角色维度优先于产品类别维度(self) -> None:
+        snap = _snap([("some.param", "municipal_water", "A"), ("some.param", "salesperson", "B")])
+        got = snap.get("some.param", role_code="salesperson", product_category="municipal_water")
+        assert got == "B"
+
+    def test_无任何维度匹配且无默认行_回退注册表默认值(self) -> None:
+        snap = _snap([("enterprise.rate", "municipal_water", "2")])
+        # 轨交无行、无默认行 → 回退注册表 default_value（3）
+        assert snap.fraction("enterprise.rate", product_category="rail_transit") == Fraction(3)
+
+
+class TestFallback:
+    """注册表默认值兜底（种子缺失时的安全网）。"""
+
+    def test_未定义参数按注册表默认值兜底(self) -> None:
+        snap = _snap([])
+        assert snap.fraction("prepay.rate") == Fraction(10)
+
+    def test_未注册编码抛ValueError_属代码缺陷(self) -> None:
+        snap = _snap([])
+        with pytest.raises(ValueError):
+            snap.fraction("not.registered.code")
+
+
+class TestSameDayChanges:
+    """同日多次变更：仓储 SQL 已按 (effective_date, id) 倒序，快照取先出现者。"""
+
+    def test_同日多次变更取最新一条(self) -> None:
+        snap = _snap([("enterprise.rate", "", "3"), ("enterprise.rate", "", "2")])
+        assert snap.fraction("enterprise.rate") == Fraction(3)
+
+
+class TestTypedGetters:
+    """类型化取值：Fraction 精确解析、整数、布尔、文本。"""
+
+    def test_fraction精确解析分数式(self) -> None:
+        snap = _snap([("salesperson.rate.slope", "salesperson", "4/360")])
+        assert snap.fraction("salesperson.rate.slope", role_code="salesperson") == Fraction(1, 90)
+
+    def test_integer(self) -> None:
+        snap = _snap([("salesperson.days.boundary", "salesperson", "360")])
+        assert snap.integer("salesperson.days.boundary", role_code="salesperson") == 360
+
+    def test_text返回枚举原文(self) -> None:
+        snap = _snap([("payment.weight.mode", "", "FULL")])
+        assert snap.text("payment.weight.mode") == "FULL"
+
+    def test_boolean(self) -> None:
+        snap = _snap([("some.flag", "", "1")])
+        assert snap.boolean("some.flag") is True
+        assert snap.boolean("some.flag") is not False
+
+
+class TestImmutability:
+    """快照冻结：解析结果不得被调用方改写（核算期间一致性）。"""
+
+    def test_字段不可赋值(self) -> None:
+        snap = _snap([])
+        with pytest.raises(FrozenInstanceError):
+            snap.biz_date = date(2026, 1, 1)  # type: ignore[misc]