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/src/lib.rs

@@ -155,6 +155,19 @@ impl KindSchema {
             .collect()
     }
 
+    /// The names of the **required** content fields (those in the schema's
+    /// `required` list), in schema order. The `cph-cli add` authoring surface
+    /// uses this to scaffold the sibling `<field>.typ` files a new part must
+    /// have (ADR-0008): an optional content field (e.g. a lemma's `proof`) is
+    /// not created, so a freshly added part stays schema-legal.
+    pub fn required_content_field_names(&self) -> Vec<&str> {
+        self.content_fields
+            .iter()
+            .filter(|f| f.required)
+            .map(|f| f.name.as_str())
+            .collect()
+    }
+
     /// The names of the scalar fields (those living in `element.toml`), in
     /// schema order.
     pub fn scalar_field_names(&self) -> Vec<&str> {