feat: defer 延迟执行 — defer { stmts; } 在 return 前按 LIFO 执行

Token(73): +TOK_DEFER, AST(28): +AST_DEFER_STMT, 新增 38_defer.l
parser: defer { ... } 块 + defer expr; 表达式两种形式
codegen: defer 栈压入 block, emit_deferred() 在 return 前 LIFO 发射

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/sema/sema.c

@@ -472,6 +472,9 @@ void analyze_node(AstNode* node, Scope* scope, ErrorList* errors, Arena* a) {
     case AST_EXPR_STMT:
         analyze_expr(node->as.expr_stmt.expr, scope, errors, a);
         break;
+    case AST_DEFER_STMT:
+        analyze_node(node->as.defer_stmt.body, scope, errors, a);
+        break;
 
     default:
         analyze_expr(node, scope, errors, a);