fix(fortran): 修复 JSON 输出缺少逗号导致解析失败

write_json 中 bond_rest_lengths 后面缺少逗号，
导致 JSON 解码器在 driving_force 前报错：
  JSONDecodeError: Expecting ',' delimiter
将 has_next 从 .false. 改为 .true.，空数组版本也补上逗号。

engines/fortran/main.f90

@@ -1042,11 +1042,11 @@ subroutine write_json(outdir, tx, ty, tz, tvx, tvy, tvz, &
     if (nb > 0) then
         call write_int2_arr(u, 'bond_pairs', bp, nb, .true.)
         call write_dbl_arr(u, 'bond_stiffness', bk, nb, .true.)
-        call write_dbl_arr(u, 'bond_rest_lengths', br, nb, .false.)
+        call write_dbl_arr(u, 'bond_rest_lengths', br, nb, .true.)
     else
         write(u, '(a)') '  "bond_pairs": [],'
         write(u, '(a)') '  "bond_stiffness": [],'
-        write(u, '(a)') '  "bond_rest_lengths": []'
+        write(u, '(a)') '  "bond_rest_lengths": [],'
     end if
 
     write(buf, '(a, i0)') '  "driving_force": ', driving_force