feat: trait 接口系统 — trait Show { fn method } + extend Trait Struct { }

src/ast/ast.c

@@ -274,3 +274,11 @@ AstNode* ast_make_use_decl(void* alloc, const char* path, const char* item, Sour
     n->as.use_decl.item = item;
     return n;
 }
+
+AstNode* ast_make_trait_decl(void* alloc, const char* name, AstNode** methods, size_t count, SourceLoc loc) {
+    NEW(alloc, AST_TRAIT_DECL);
+    n->as.trait_decl.name = name;
+    n->as.trait_decl.methods = methods;
+    n->as.trait_decl.method_count = count;
+    return n;
+}