feat: 类型别名 type alias (P1 #10)

- lexer: TOK_TYPE 关键字
- ast: AST_TYPE_ALIAS + AST_PROGRAM aliases数组
- parser: parse_type_expr() 抽取, type Name = Type; 解析
- sema: 别名注册+解析, 类型标注/struct字段/函数参数均支持
- 新增测试: 15_type_alias.l, 16_type_alias_struct.l

测试: 112 通过 (41+15+41+15)

src/ast/ast.h

@@ -23,6 +23,7 @@ typedef enum {
     AST_STRUCT_DECL,      // struct Point { x: i64, y: i64 }
     AST_STRUCT_INIT,      // Point { x: 10, y: 20 }
     AST_FIELD_ACCESS,     // p.x
+    AST_TYPE_ALIAS,       // type Meters = i64
 } AstKind;
 
 typedef enum {
@@ -48,7 +49,8 @@ struct AstNode {
     union {
         // AST_PROGRAM
         struct { struct AstNode** functions; size_t fn_count;
-                 struct AstNode** structs;   size_t struct_count; } program;
+                 struct AstNode** structs;   size_t struct_count;
+                 struct AstNode** type_aliases; size_t alias_count; } program;
         // AST_FUNCTION
         struct { const char* name; struct AstNode** params; size_t param_count;
                  TypeKind return_type; const char* return_struct_type_name;
@@ -87,12 +89,15 @@ struct AstNode {
                  struct AstNode** field_values; size_t field_count; } struct_init;
         // AST_FIELD_ACCESS
         struct { struct AstNode* object; const char* field; int field_index; } field_access;
+        // AST_TYPE_ALIAS
+        struct { const char* name; TypeKind aliased_type; const char* aliased_struct_name; } type_alias;
     } as;
 };
 
 // 创建节点的辅助函数（内存来自 arena，通过 void* 传递避免循环依赖）
 AstNode* ast_make_program(void* alloc, AstNode** fns, size_t fn_count,
-                          AstNode** structs, size_t struct_count, SourceLoc loc);
+                          AstNode** structs, size_t struct_count,
+                          AstNode** aliases, size_t alias_count, SourceLoc loc);
 AstNode* ast_make_function(void* alloc, const char* name, AstNode** params, size_t pcount,
                            TypeKind ret, const char* ret_struct_name, AstNode* body, SourceLoc loc);
 AstNode* ast_make_parameter(void* alloc, const char* name, TypeKind type, const char* struct_type_name, SourceLoc loc);
@@ -115,5 +120,7 @@ AstNode* ast_make_ident(void* alloc, const char* name, SourceLoc loc);
 AstNode* ast_make_struct_decl(void* alloc, const char* name, AstNode** fields, size_t count, SourceLoc loc);
 AstNode* ast_make_struct_init(void* alloc, const char* type_name, const char** fnames, AstNode** fvals, size_t count, SourceLoc loc);
 AstNode* ast_make_field_access(void* alloc, AstNode* object, const char* field, SourceLoc loc);
+AstNode* ast_make_type_alias(void* alloc, const char* name, TypeKind aliased,
+                              const char* aliased_struct, SourceLoc loc);
 
 #endif