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/codegen/codegen.c

@@ -63,6 +63,7 @@ static LLVMTypeRef to_llvm_type(CgCtx* ctx, TypeKind kind) {
         case TYPE_BOOL: return LLVMInt1TypeInContext(ctx->context);
         case TYPE_STR:  return LLVMPointerType(LLVMInt8TypeInContext(ctx->context), 0);
         case TYPE_STRUCT:
+        case TYPE_ENUM:  return LLVMInt64TypeInContext(ctx->context);
         case TYPE_UNKNOWN:
         case TYPE_ERROR:
         default:        return LLVMVoidTypeInContext(ctx->context);
@@ -347,6 +348,10 @@ static LLVMValueRef codegen_expr(CgCtx* ctx, AstNode* node) {
         return LLVMBuildLoad2(ctx->builder, struct_ty, alloca, "struct_val");
     }
 
+    case AST_ENUM_VARIANT:
+        return LLVMConstInt(LLVMInt64TypeInContext(ctx->context),
+            (unsigned long long)node->as.enum_variant.variant_index, true);
+
     default:
         return NULL;
     }