目录整理：RTL/上板/仿真分开存放，测试 ROM 镜像归入 testbench/rom

- rtl/：12 个可综合的处理器源文件（顶层 cpu21_riscv_redirect_int_bpb.v，其余为各功能单元）。
- board/：板级顶层 cpu21_board_top.v、引脚约束 cpu21_board_top.xdc、片内 ROM 初始化
  cpu21_board_rom.vh 与短延时版本 cpu21_board_rom_fast.vh。
- testbench/：保留三个测试平台；ROM 镜像统一放到 testbench/rom/，测试程序源码与期望值
  仍在 testbench/programs/（其中的 .hex 是 RARS 原始输出，与 rom/ 里改过显示方式的版本
  内容不同，两者都保留）。
- 同步更新：两个测试平台的 ROM 探测路径改为 testbench/rom/（并在头注释里补充说明：
  PowerShell 会把 -generic_top/--testplusarg 参数里的 "=" 拆开、Verilog 字符串中的
  路径要用正斜杠）；tools/hex2vh.py 的用法示例指向 board/cpu21_board_top.v；
  说明文档新增“目录结构”一节并同步所有路径引用；.gitignore 补充 .Xil/。
- 清理：删除 *.hex.lst 中间文件与空的 .Xil/ 目录。
- 验证：isa_test 24/24、lui_test 4/4、mmio_test 5/5 三项回归通过；两个基线（5560 / 6003
  周期）与整理前一致；板级仿真（默认 ROM 首次写入 LED=0001、快速 ROM 跑马灯循环）正常；
  Vivado 工程源文件路径同步到 rtl/ 与 board/ 后综合通过（LUT 5276、23 个引脚，与整理前
  完全一致），因此已有 bit 文件仍然有效。

testbench/rom/risc-v-benchmark_ccab.hex

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