feat: ?? 空值合并运算符 — a ?? b → if a!=0 {a} else {b}

Token(76): +TOK_QMARK_QMARK, 优先级 PREC_COALESCE=15 (||之上, |>之下)
parser 去糖为 if-expr, 零 sema/codegen 改动

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

src/lexer/token.c

@@ -30,6 +30,7 @@ static const char* NAMES[] = {
     [TOK_COMMA] = ",", [TOK_COLON] = ":", [TOK_SEMICOLON] = ";",
     [TOK_ASSIGN] = "=",
     [TOK_DOT] = ".", [TOK_COLON_COLON] = "::", [TOK_HASH] = "#",
+    [TOK_QMARK_QMARK] = "??",
     [TOK_EOF] = "EOF", [TOK_ERROR] = "错误",
 };