feat(impact): Impact Agent MVP —— 变更点定位 + 影响调查书（追加改修场景）

- 门控：用户提供 existing_system 路径 → 进入影响调查；未提供 → 原流程不变
- CodeParser 解析 Java（@RestController/@Service/@Entity/@TableName）+ ExistingSystemExplorer 组装
- ImpactAgent 变更点定位（变更区分×既存対応 确定性比对，无 LLM）→ ImpactReport（JSON 可下载）
- 影响调查结果作为 Writer 生成概要设计书的主上下文（{{impact}}，无专用影响章）
- source_aggregator 解除 existing_system=None 硬编码
- 既有系统样本 sunOnly/stock-trade-system（无 LICENSE，仅测试输入，保留来源标注）
- 新造股票交易域追加改修样本 要件定義_追加改修_股票.xlsx（对齐 sunOnly 真实类名）
- 全量 351 passed / 99.27% 覆盖；门禁 PASS（16 要素：新规5/変更8/削除3/未受影响50）

tests/test_source_aggregator.py

@@ -2,6 +2,7 @@ import pytest
 from pathlib import Path
 
 from genesis.data_models import StructuredSource
+from genesis.impact.code_parser import CodeParseError
 from genesis.parsers.source_aggregator import SourceParser
 
 from tests.docx_helpers import make_rule_doc, new_document, save_document
@@ -125,3 +126,76 @@ def test_validate_path_returns_resolved_path(tmp_path):
     result = _validate_path(str(good), (".xlsx",))
     assert isinstance(result, Path)
     assert result.suffix.lower() == ".xlsx"
+
+
+# ---------- Impact Agent MVP：existing_system_path（2026-08-23） ----------
+
+JAVA_PROJECT = {
+    "trade-order/OrderController.java": (
+        "package com.trade.order.controller;\n"
+        "import org.springframework.web.bind.annotation.*;\n"
+        "@RestController\n"
+        "@RequestMapping(\"/api/order\")\n"
+        "public class OrderController {\n"
+        "    @GetMapping(\"/{id}\")\n"
+        "    public String get(Long id) { return \"ok\"; }\n"
+        "}\n"
+    ),
+    "trade-order/OrderServiceImpl.java": (
+        "package com.trade.order.service;\n"
+        "import org.springframework.stereotype.Service;\n"
+        "@Service\n"
+        "public class OrderServiceImpl {\n"
+        "    public void createOrder() {}\n"
+        "}\n"
+    ),
+    "trade-order/OrderDO.java": (
+        "package com.trade.order.entity;\n"
+        "import com.baomidou.mybatisplus.annotation.TableName;\n"
+        "@TableName(\"trade_order\")\n"
+        "public class OrderDO {\n"
+        "    private Long id;\n"
+        "}\n"
+    ),
+}
+
+
+def _java_project(tmp_path) -> str:
+    for rel, content in JAVA_PROJECT.items():
+        p = tmp_path / "existing" / rel
+        p.parent.mkdir(parents=True, exist_ok=True)
+        p.write_text(content, encoding="utf-8")
+    return str(tmp_path / "existing")
+
+
+def test_parse_with_existing_system_path(tmp_path):
+    xlsx = _xlsx(tmp_path)
+    existing = _java_project(tmp_path)
+
+    result = SourceParser().parse(requirement_paths=[xlsx], existing_system_path=existing)
+
+    assert result.existing_system is not None
+    names = {c.class_name for c in result.existing_system.controller_layer}
+    assert "OrderController" in names
+    assert {s.class_name for s in result.existing_system.service_layer} == {"OrderServiceImpl"}
+    assert {e.class_name for e in result.existing_system.entity_layer} == {"OrderDO"}
+
+
+def test_parse_without_existing_system_path_keeps_none(tmp_path):
+    xlsx = _xlsx(tmp_path)
+    result = SourceParser().parse(requirement_paths=[xlsx])
+    assert result.existing_system is None
+
+
+def test_parse_existing_system_path_invalid_dir_raises(tmp_path):
+    xlsx = _xlsx(tmp_path)
+    with pytest.raises(CodeParseError):
+        SourceParser().parse(requirement_paths=[xlsx], existing_system_path=tmp_path / "nope")
+
+
+def test_parse_existing_system_path_without_java_raises(tmp_path):
+    xlsx = _xlsx(tmp_path)
+    empty = tmp_path / "empty"
+    empty.mkdir()
+    with pytest.raises(CodeParseError):
+        SourceParser().parse(requirement_paths=[xlsx], existing_system_path=empty)