add % modulo operator to El lexer, parser, codegen

Lexer and parser already had Percent token and precedence on the
compiler/string-interp branch. This commit adds the missing is_int_expr
case for Percent so that modulo expressions over Int operands are
correctly typed as Int (enabling arithmetic dispatch rather than
falling through to string concat or untyped paths).

binop_to_c already mapped Percent -> % at HEAD; only is_int_expr
needed the Percent arm.

el-compiler/src/codegen.el

@@ -1771,6 +1771,12 @@ fn is_int_expr(expr: Map<String, Any>) -> Bool {
             }
             return false
         }
+        if str_eq(op, "Percent") {
+            if is_int_expr(expr["left"]) {
+                if is_int_expr(expr["right"]) { return true }
+            }
+            return false
+        }
         return false
     }
     return false