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.c

@@ -10,7 +10,8 @@
 
 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) {
     NEW(alloc, AST_PROGRAM);
     n->as.program.functions = fns;
     n->as.program.fn_count = fn_count;
@@ -18,6 +19,8 @@ AstNode* ast_make_program(void* alloc, AstNode** fns, size_t fn_count,
     n->as.program.struct_count = struct_count;
     n->as.program.type_aliases = aliases;
     n->as.program.alias_count = alias_count;
+    n->as.program.enums = enums;
+    n->as.program.enum_count = enum_count;
     return n;
 }
 
@@ -177,3 +180,23 @@ AstNode* ast_make_type_alias(void* alloc, const char* name, TypeKind aliased,
     n->as.type_alias.aliased_struct_name = aliased_struct;
     return n;
 }
+
+// === 枚举相关工厂函数 ===
+
+AstNode* ast_make_enum_decl(void* alloc, const char* name, const char** variants,
+                              size_t count, SourceLoc loc) {
+    NEW(alloc, AST_ENUM_DECL);
+    n->as.enum_decl.name = name;
+    n->as.enum_decl.variants = variants;
+    n->as.enum_decl.variant_count = count;
+    return n;
+}
+
+AstNode* ast_make_enum_variant(void* alloc, const char* enum_name,
+                                 const char* variant_name, SourceLoc loc) {
+    NEW(alloc, AST_ENUM_VARIANT);
+    n->as.enum_variant.enum_name = enum_name;
+    n->as.enum_variant.variant_name = variant_name;
+    n->as.enum_variant.variant_index = -1;
+    return n;
+}