feat: 枚举 enum (P1 #7)

- lexer: TOK_ENUM, TOK_COLON_COLON
- ast: AST_ENUM_DECL, AST_ENUM_VARIANT + AST_PROGRAM enums数组
- parser: enum Name { A, B } + Enum::Variant语法
- sema: SYM_ENUM, 变体验证, enum→i64类型兼容
- codegen: TYPE_ENUM→i64, 变体→ConstInt(索引值)
- 新增集成测试: 17_enum.l

测试: 121 通过 (41+15+47+18)

test/test_sema.c

@@ -223,6 +223,42 @@ void test_struct_nested_type_ok() {
     arena_destroy(&a);
 }
 
+/* === enum 语义分析测试 === */
+
+void test_enum_ok() {
+    Arena a = arena_create(1);
+    size_t tc; ErrorInfo lex_err = {0};
+    Token* toks = lex(&a,
+        "enum Color { Red, Green, Blue } fn main() { let c = Color::Green; print_i64(c); return; }",
+        "test", &tc, &lex_err);
+    ASSERT(toks != NULL);
+    ErrorInfo parse_err = {0};
+    AstNode* ast = parse(&a, toks, tc, "test", &parse_err);
+    ASSERT(ast != NULL);
+
+    ErrorList errors; error_init(&errors);
+    sema_analyze(ast, &errors, &a);
+    ASSERT(errors.count == 0);
+    arena_destroy(&a);
+}
+
+void test_enum_bad_variant() {
+    Arena a = arena_create(1);
+    size_t tc; ErrorInfo lex_err = {0};
+    Token* toks = lex(&a,
+        "enum Color { Red, Green, Blue } fn main() { let c = Color::Yellow; return; }",
+        "test", &tc, &lex_err);
+    ASSERT(toks != NULL);
+    ErrorInfo parse_err = {0};
+    AstNode* ast = parse(&a, toks, tc, "test", &parse_err);
+    ASSERT(ast != NULL);
+
+    ErrorList errors; error_init(&errors);
+    sema_analyze(ast, &errors, &a);
+    ASSERT(errors.count > 0);  // Yellow 不存在
+    arena_destroy(&a);
+}
+
 int main(void) {
     TEST_RUN(test_type_error);
     TEST_RUN(test_undefined_var);
@@ -237,5 +273,7 @@ int main(void) {
     TEST_RUN(test_struct_nested_type_ok);
     TEST_RUN(test_type_alias_ok);
     TEST_RUN(test_type_alias_struct);
+    TEST_RUN(test_enum_ok);
+    TEST_RUN(test_enum_bad_variant);
     return test_summary();
 }