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)

src/ast/ast.h

@@ -24,6 +24,8 @@ typedef enum {
     AST_STRUCT_INIT,      // Point { x: 10, y: 20 }
     AST_FIELD_ACCESS,     // p.x
     AST_TYPE_ALIAS,       // type Meters = i64
+    AST_ENUM_DECL,        // enum Color { Red, Green, Blue }
+    AST_ENUM_VARIANT,     // Color::Red
 } AstKind;
 
 typedef enum {
@@ -50,7 +52,8 @@ struct AstNode {
         // AST_PROGRAM
         struct { struct AstNode** functions; size_t fn_count;
                  struct AstNode** structs;   size_t struct_count;
-                 struct AstNode** type_aliases; size_t alias_count; } program;
+                 struct AstNode** type_aliases; size_t alias_count;
+                 struct AstNode** enums;     size_t enum_count; } program;
         // AST_FUNCTION
         struct { const char* name; struct AstNode** params; size_t param_count;
                  TypeKind return_type; const char* return_struct_type_name;
@@ -91,13 +94,18 @@ struct AstNode {
         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;
+        // AST_ENUM_DECL
+        struct { const char* name; const char** variants; size_t variant_count; } enum_decl;
+        // AST_ENUM_VARIANT
+        struct { const char* enum_name; const char* variant_name; int variant_index; } enum_variant;
     } 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,
-                          AstNode** aliases, size_t alias_count, SourceLoc loc);
+                          AstNode** aliases, size_t alias_count,
+                          AstNode** enums, size_t enum_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);
@@ -122,5 +130,7 @@ AstNode* ast_make_struct_init(void* alloc, const char* type_name, const char** f
 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);
+AstNode* ast_make_enum_decl(void* alloc, const char* name, const char** variants, size_t count, SourceLoc loc);
+AstNode* ast_make_enum_variant(void* alloc, const char* enum_name, const char* variant_name, SourceLoc loc);
 
 #endif