feat(writer): docx 注入原型提前（T17 架构审查整改，OV8）

- T17 (OV8, P1): 新建 src/genesis/writer/ 包
  - docx_injector.py: DocxInjector 用原生 python-docx 实现 §6.6 占位符注入
    - 章节级 {{section:id}} → 内容块 docx 元素序列（heading/paragraph/table）
    - 行内 {{meta}} → 元信息填充
    - 残留检查: 未替换 {{...}} 抛 DocxInjectError
    - 格式精度: 注入 heading 继承模板 Heading 样式，原内容不被破坏
  - 新增 test_docx_injector.py（5 用例）
  - 同步 design.md §6.7 渲染链路 T17 原型说明
- TDD: RED（模块缺失）→ GREEN（聚焦 5 passed）→ 全量 245 passed / 100.00%（1361 stmts/340 br）

tests/test_docx_injector.py

@@ -0,0 +1,89 @@
+"""docx 注入原型测试（T17，OV8）。
+
+OV8 裁定：最难成功标准（格式精度）排关键路径末尾 → docx 注入原型提前验证。
+本文件测试 DocxInjector：模板占位符替换（章节级/行内）、残留检查、格式精度
+（注入 heading 继承模板 Heading 样式，原有内容样式不被破坏）。
+"""
+
+from __future__ import annotations
+
+import pytest
+from docx import Document
+
+from genesis.writer.docx_injector import (
+    Block,
+    DocxInjectError,
+    DocxInjector,
+)
+
+
+def _make_template(tmp_path, body: str) -> str:
+    doc = Document()
+    doc.add_paragraph("{{doc_title}}")  # 行内占位符
+    doc.add_paragraph(body)            # 章节占位符所在段落
+    doc.add_paragraph("尾部固定内容")
+    path = tmp_path / "template.docx"
+    doc.save(str(path))
+    return str(path)
+
+
+def _section_blocks() -> list[Block]:
+    return [
+        Block(kind="heading", text="3.1 テーブル一覧", level=2),
+        Block(kind="paragraph", text="以下がDB表定义です。"),
+        Block(kind="table", rows=[["テーブル", "説明"], ["TB001", "社員"]]),
+    ]
+
+
+# ---------- 章节级占位符替换 ----------
+
+def test_section_placeholder_replaced(tmp_path):
+    tpl = _make_template(tmp_path, "{{section:db_design}}")
+    inj = DocxInjector(tpl)
+    out = inj.inject({"db_design": _section_blocks()}, {"doc_title": "概要設計書"})
+
+    full_text = "\n".join(p.text for p in out.paragraphs)
+    assert "{{section:db_design}}" not in full_text
+    assert "3.1 テーブル一覧" in full_text
+    assert "以下がDB表定义です。" in full_text
+
+
+# ---------- 行内占位符替换 ----------
+
+def test_inline_meta_replaced(tmp_path):
+    tpl = _make_template(tmp_path, "{{section:db_design}}")
+    inj = DocxInjector(tpl)
+    out = inj.inject({"db_design": []}, {"doc_title": "概要設計書"})
+
+    assert "{{doc_title}}" not in "\n".join(p.text for p in out.paragraphs)
+    assert "概要設計書" in "\n".join(p.text for p in out.paragraphs)
+
+
+# ---------- 占位符残留检查 ----------
+
+def test_residue_detection_raises(tmp_path):
+    tpl = _make_template(tmp_path, "{{section:unknown_chapter}}")
+    inj = DocxInjector(tpl)
+    with pytest.raises(DocxInjectError, match="残留"):
+        inj.inject({}, {"doc_title": "X"})
+
+
+# ---------- 格式精度：heading 继承模板样式 ----------
+
+def test_heading_inherits_template_style(tmp_path):
+    tpl = _make_template(tmp_path, "{{section:db_design}}")
+    inj = DocxInjector(tpl)
+    out = inj.inject({"db_design": _section_blocks()}, {"doc_title": "T"})
+
+    # 注入的 heading block（level=2）应渲染为模板中存在的 Heading 2 样式段落
+    heading_paras = [p for p in out.paragraphs if p.style.name == "Heading 2"]
+    assert any("3.1 テーブル一覧" in p.text for p in heading_paras)
+
+
+def test_original_content_style_preserved(tmp_path):
+    tpl = _make_template(tmp_path, "{{section:db_design}}")
+    inj = DocxInjector(tpl)
+    out = inj.inject({"db_design": []}, {"doc_title": "T"})
+
+    # 模板原有段落（尾部固定内容）在注入后仍存在且未被破坏
+    assert any("尾部固定内容" in p.text for p in out.paragraphs)