feat(cli): cph init / cph add 子命令(工程脚手架)

init 生成可 check 的工程根(manifest.toml + .cph-version + 默认 exports/student.typ + 空 kind 目录);
add 按 kind 建 part 目录/element.toml/必填内容文件并追加 [[parts]]。均纯本地,不涉及 hub 语义;
Engine 改惰性构造。cph-schema 新增 required_content_field_names() 作为 add 建文件合同。

crates/cph-schema/tests/validate.rs

@@ -74,3 +74,25 @@ fn example_source_non_string_is_schema_violation() {
     assert!(diags[0].message.contains("source"));
     assert!(diags[0].message.contains("string"));
 }
+
+/// `required_content_field_names` is the `cph-cli add` authoring contract: the
+/// set of sibling `.typ` files a new part of a kind must have to be
+/// schema-legal (ADR-0008). It must be the schema `required` content fields —
+/// not the optional ones (e.g. a lemma's `proof`) and not the scalar fields.
+#[test]
+fn required_content_field_names_match_schema_required() {
+    let case = |kind: &str, expected: Vec<&str>| {
+        let mut got: Vec<&str> = cph_schema::schema_for(kind)
+            .unwrap()
+            .required_content_field_names();
+        got.sort_unstable();
+        let mut want = expected;
+        want.sort_unstable();
+        assert_eq!(got, want, "required content fields for '{kind}'");
+    };
+
+    case("segment", vec!["textbook"]);
+    case("lemma", vec!["stmt"]);
+    case("example", vec!["problem", "solution"]);
+    case("sop", vec!["sop"]);
+}