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/parser/desugar.c

@@ -39,25 +39,23 @@ AstNode* desugar_guard(Parser* p, const Token* guard_tok,
     return ast_make_if(p->arena, not_cond, body, NULL, tok_loc(guard_tok));
 }
 
-// === for i in start to end { body } → { var i = start; while i < end { body; i = i + 1; } } ===
+// === for i in start to end [step N] { body } → { var i=start; while i<end { body; i=i+step; } } ===
 AstNode* desugar_for(Parser* p, const Token* for_tok,
                      const char* var_name, AstNode* start_expr,
-                     AstNode* end_expr, AstNode* body) {
-    // var i = start;
+                     AstNode* end_expr, AstNode* step_expr, AstNode* body) {
     AstNode* let_stmt = ast_make_let(p->arena, var_name, TYPE_UNKNOWN,
         false, true, start_expr, NULL, 0, NULL, 0, tok_loc(for_tok));
 
-    // i < end
     AstNode* cond = ast_make_binary(p->arena, OP_LT,
         ast_make_ident(p->arena, var_name, tok_loc(for_tok)),
         end_expr, tok_loc(for_tok));
 
-    // i = i + 1
+    AstNode* step = step_expr ? step_expr
+        : ast_make_literal_i64(p->arena, 1, tok_loc(for_tok));
     AstNode* incr = ast_make_assign(p->arena, var_name,
         ast_make_binary(p->arena, OP_ADD,
             ast_make_ident(p->arena, var_name, tok_loc(for_tok)),
-            ast_make_literal_i64(p->arena, 1, tok_loc(for_tok)),
-            tok_loc(for_tok)),
+            step, tok_loc(for_tok)),
         tok_loc(for_tok));
 
     // 增量追加到循环体末尾