feat: for step 步长 — for i in 0 to 10 step 2 { ... }

Token(75): +TOK_STEP, desugar_for 增加 step_expr 参数, 默认 step=1

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

src/lexer/token.c

@@ -22,7 +22,7 @@ static const char* NAMES[] = {
     [TOK_EQ_EQ] = "==", [TOK_BANG_EQ] = "!=",
     [TOK_LT] = "<", [TOK_GT] = ">", [TOK_LT_EQ] = "<=", [TOK_GT_EQ] = ">=",
     [TOK_AND_AND] = "&&", [TOK_PIPE_PIPE] = "||", [TOK_PIPE] = "|>", [TOK_BANG] = "!",
-    [TOK_ARROW] = "->", [TOK_TO] = "to", [TOK_MATCH_ARROW] = "=>",
+    [TOK_ARROW] = "->", [TOK_TO] = "to", [TOK_STEP] = "step", [TOK_MATCH_ARROW] = "=>",
     [TOK_PLUS_EQ] = "+=", [TOK_MINUS_EQ] = "-=", [TOK_STAR_EQ] = "*=", [TOK_SLASH_EQ] = "/=",
     [TOK_LPAREN] = "(", [TOK_RPAREN] = ")",
     [TOK_LBRACE] = "{", [TOK_RBRACE] = "}",