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/lexer/token.c

@@ -10,7 +10,7 @@ static const char* NAMES[] = {
     [TOK_TRAIT] = "trait", [TOK_SELF] = "Self",
     [TOK_ELSE] = "else", [TOK_WHILE] = "while", [TOK_FOR] = "for", [TOK_IN] = "in", [TOK_RETURN] = "return",
     [TOK_STRUCT] = "struct", [TOK_TYPE] = "type", [TOK_ENUM] = "enum", [TOK_EXTEND] = "extend",
-    [TOK_MATCH] = "match",
+    [TOK_DEFER] = "defer", [TOK_MATCH] = "match",
     [TOK_I32] = "i32", [TOK_I64] = "i64", [TOK_U64] = "u64", [TOK_F64] = "f64",
     [TOK_BOOL] = "bool", [TOK_CHAR] = "char", [TOK_STR] = "str", [TOK_VOID] = "void",
     [TOK_INT_LIT] = "整数", [TOK_FLOAT_LIT] = "浮点数",