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_template_mapper.py

@@ -42,3 +42,46 @@ def test_map_template_section_re_case_insensitive():
     specs = map_template(parsed)
     assert specs[0].chapter_id == "2"
     assert specs[0].section_placeholder == "Section:2"
+
+
+# ---------- design.md §6.5：H1 起章，H2/H3 归入本章作为子节（不再独立成章被丢弃） ----------
+
+def test_h2_headings_merge_into_parent_chapter():
+    sections = [
+        ChapterMarker(type="heading", name="2. 機能一覧", level=1),
+        ChapterMarker(type="placeholder", name="section:function_list", level=0),
+        ChapterMarker(type="heading", name="2.1 機能一覧表", level=2),
+        ChapterMarker(type="heading", name="2.2 機能詳細", level=2),
+    ]
+    parsed = ParsedTemplate(file_name="t.docx", sections=sections, placeholders={}, styles={})
+    specs = map_template(parsed)
+    assert len(specs) == 1
+    s = specs[0]
+    assert s.chapter_id == "function_list"
+    assert s.title == "2. 機能一覧"
+    assert s.sub_headings == ["2.1 機能一覧表", "2.2 機能詳細"]
+
+
+def test_h3_headings_also_merge():
+    sections = [
+        ChapterMarker(type="heading", name="5. DB設計", level=1),
+        ChapterMarker(type="placeholder", name="section:db_design", level=0),
+        ChapterMarker(type="heading", name="5.1 テーブル一覧", level=2),
+        ChapterMarker(type="heading", name="5.1.1 補足", level=3),
+    ]
+    parsed = ParsedTemplate(file_name="t.docx", sections=sections, placeholders={}, styles={})
+    specs = map_template(parsed)
+    assert len(specs) == 1
+    assert specs[0].sub_headings == ["5.1 テーブル一覧", "5.1.1 補足"]
+
+
+def test_leading_h2_without_parent_starts_chapter():
+    """异常模板防御：任何 H1 之前出现 H2 → 回落为独立章（不崩溃、不静默丢弃）。"""
+    sections = [
+        ChapterMarker(type="heading", name="孤立节", level=2),
+        ChapterMarker(type="heading", name="1. はじめに", level=1),
+    ]
+    parsed = ParsedTemplate(file_name="t.docx", sections=sections, placeholders={}, styles={})
+    specs = map_template(parsed)
+    assert [s.title for s in specs] == ["孤立节", "1. はじめに"]
+    assert specs[0].sub_headings == []