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)

test/test_sema.c

@@ -168,6 +168,44 @@ void test_struct_field_count_mismatch() {
     arena_destroy(&a);
 }
 
+/* === 类型别名测试 === */
+
+void test_type_alias_ok() {
+    Arena a = arena_create(1);
+    size_t tc; ErrorInfo lex_err = {0};
+    Token* toks = lex(&a,
+        "type Meters = i64; fn main() { let x: Meters = 100; print_i64(x); return; }",
+        "test", &tc, &lex_err);
+    ASSERT(toks != NULL);
+    ErrorInfo parse_err = {0};
+    AstNode* ast = parse(&a, toks, tc, "test", &parse_err);
+    ASSERT(ast != NULL);
+    ASSERT(ast->as.program.alias_count == 1);
+
+    ErrorList errors; error_init(&errors);
+    sema_analyze(ast, &errors, &a);
+    ASSERT(errors.count == 0);
+    arena_destroy(&a);
+}
+
+void test_type_alias_struct() {
+    Arena a = arena_create(1);
+    size_t tc; ErrorInfo lex_err = {0};
+    Token* toks = lex(&a,
+        "struct Point { x: i64, y: i64 } type P = Point; fn main() { let p: P = Point { x: 1, y: 2 }; return; }",
+        "test", &tc, &lex_err);
+    ASSERT(toks != NULL);
+    ErrorInfo parse_err = {0};
+    AstNode* ast = parse(&a, toks, tc, "test", &parse_err);
+    ASSERT(ast != NULL);
+    ASSERT(ast->as.program.alias_count == 1);
+
+    ErrorList errors; error_init(&errors);
+    sema_analyze(ast, &errors, &a);
+    ASSERT(errors.count == 0);
+    arena_destroy(&a);
+}
+
 void test_struct_nested_type_ok() {
     Arena a = arena_create(1);
     size_t tc; ErrorInfo lex_err = {0};
@@ -197,5 +235,7 @@ int main(void) {
     TEST_RUN(test_struct_undefined);
     TEST_RUN(test_struct_field_count_mismatch);
     TEST_RUN(test_struct_nested_type_ok);
+    TEST_RUN(test_type_alias_ok);
+    TEST_RUN(test_type_alias_struct);
     return test_summary();
 }