fix(writer): 无锚点子章归并父章生成 + 注入后裸重复子节去重

- template_mapper: 按 design §6.5 仅 Heading level<=1 起章，H2/H3 归入
  父章 sub_headings（此前每个 heading 独立成章，无 {{section:id}} 锚点的
  6 个子章生成后被静默丢弃）
- models: ChapterSpec.sub_headings 字段；to_vars 暴露 sub_headings 变量
- writer_agent: prompt 新增【小节约束】（有子节时按小节顺序以 level=2
  heading 组织，不得遗漏或新增）
- docx_injector: 注入后删除同章内与已生成标题同名且完全无内容的模板裸
  H2/H3（保守策略：带内容的模板子节保留）
- 真实试运行验证：7 章 / 无静默丢弃告警 / 14 个 H2 无重复

tests/test_phase5_models.py

@@ -213,3 +213,32 @@ def test_impact_introduction_shows_all_elements():
     report = ImpactReport(metadata={}, change_analysis=ca, summary={"new": 2})
     impact = _ctx("introduction", "はじめに", source=None, report=report).to_vars()["impact"]
     assert "T001" in impact and "F001" in impact
+
+
+# ---------- 子节结构注入（design.md §6.5：H2/H3 归入本章） ----------
+
+def test_chapter_spec_sub_headings_default_empty():
+    s = ChapterSpec(chapter_id="x", title="X")
+    assert s.sub_headings == []
+
+
+def test_to_vars_exposes_sub_headings():
+    spec = ChapterSpec(
+        chapter_id="function_list", title="2. 機能一覧",
+        section_placeholder="section:function_list",
+        sub_headings=["2.1 機能一覧表", "2.2 機能詳細"],
+    )
+    ctx = GenerationContext(
+        chapter_id=spec.chapter_id, title=spec.title,
+        template_marker=spec,
+        structured_source=None, write_rules=[], design_rules=[],
+        template_styles=set(),
+    )
+    v = ctx.to_vars()
+    assert "2.1 機能一覧表" in v["sub_headings"]
+    assert "2.2 機能詳細" in v["sub_headings"]
+
+
+def test_to_vars_sub_headings_empty_when_none():
+    vars_ = _ctx("db_design", "DB設計").to_vars()
+    assert vars_["sub_headings"] == ""