fix(cph): correct nested outline hierarchy

crates/cph-model/tests/fixtures/nested/manifest.toml

@@ -12,5 +12,10 @@ path = "segments/开场白"
 [[children]]
 kind = "section"
 path = "导言簇"
+notes = "这里先建立直观图像，再进入分组推导。"
+
+[[children]]
+kind = "section"
+path = "收束簇"
 
 [targets.student]

crates/cph-model/tests/fixtures/nested/收束簇/manifest.toml

@@ -0,0 +1,6 @@
+[group]
+title = "收束簇"
+
+[[children]]
+kind = "segment"
+path = "segments/总结"

crates/cph-model/tests/fixtures/nested/收束簇/segments/总结/element.toml

@@ -0,0 +1 @@
+kind = "segment"

crates/cph-model/tests/fixtures/nested/收束簇/segments/总结/textbook.typ

@@ -0,0 +1 @@
+总结

crates/cph-model/tests/load.rs

@@ -121,6 +121,7 @@ fn nested_sections_flatten_depth_first_with_correct_depths() {
             PathBuf::from("导言簇/segments/子段一"),
             PathBuf::from("导言簇/嵌套子节/lemmas/子引理"),
             PathBuf::from("导言簇/segments/子段二"),
+            PathBuf::from("收束簇/segments/总结"),
         ],
         "root-relative paths must accumulate through every nesting level"
     );
@@ -139,7 +140,7 @@ fn nested_sections_flatten_depth_first_with_correct_depths() {
                 kind: "section".to_string(),
                 title: "导言簇".to_string(),
                 depth: 1,
-                notes: None,
+                notes: Some("这里先建立直观图像，再进入分组推导。".to_string()),
                 path: PathBuf::from("导言簇"),
             },
             OutlineEntry::Element {
@@ -161,8 +162,25 @@ fn nested_sections_flatten_depth_first_with_correct_depths() {
                 part_index: 3,
                 depth: 1,
             }, // 导言簇/segments/子段二
+            OutlineEntry::Section {
+                kind: "section".to_string(),
+                title: "收束簇".to_string(),
+                depth: 1,
+                notes: None,
+                path: PathBuf::from("收束簇"),
+            },
+            OutlineEntry::Element {
+                part_index: 4,
+                depth: 1,
+            }, // 收束簇/segments/总结
         ]
     );
+    let document = lesson.outline_document();
+    assert_eq!(document.children.len(), 3);
+    assert_eq!(document.children[1].title, "导言簇");
+    assert_eq!(document.children[1].children.len(), 3);
+    assert_eq!(document.children[2].title, "收束簇");
+    assert_eq!(document.children[2].children[0].title, "总结");
 
     // The outer section declares [group].title = "导言簇"; the inner section
     // has no [group] at all, so its title falls back to the folder basename.