fix(vol1): 修正 cache 步长实验量纲错误(总耗时→每次访问耗时)#68
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原版用「遍历数组的总耗时」展示 Cache 效应，但 i+=stride 使步长 翻倍访问次数减半，总耗时被访问次数主导而单调下降，cache 命中差异 被完全淹没——参考输出(68ms→223ms 上升)在任何真实机器上都复现不出来。 改用 clock_gettime(CLOCK_MONOTONIC) 测墙上时间，输出 per_access=总时间÷访问次数，剥离访问次数干扰；同步重写解释段、 踩坑预警、练习题。修正后 per_access 在 stride=16(64B 缓存行边界) 处出现拐点，与文字解释自洽。 Closes #67
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感谢 @Nyuudoukumo 老师的反馈，这里发现一个比较严重的内容错误，修改前的原版代码原版用「遍历数组的总耗时」展示 Cache 效应，但 i+=stride 使步长。翻倍访问次数减半，总耗时被访问次数主导而单调下降，cache 命中差异被完全淹没。
现在改用 clock_gettime(CLOCK_MONOTONIC) 测墙上时间，输出per_access=总时间÷访问次数，剥离访问次数干扰；同步重写解释段、踩坑预警、练习题。修正后 per_access 在 stride=16(64B 缓存行边界)处出现拐点