docs: address community issue feedback (#5 #338 #142 #246 #136 #303)

- const README: rewrite the space-saving claim per issue #5 follow-ups
  (constant-expression const vs #define immediates; which const qualify
  as constant expressions)
- const README: fix the class-section diagnosis per issue #338 (overload
  resolution picks the const overload; the restriction only surfaces
  once that overload is removed)
- sizeof README: virtual-inheritance result is compiler-dependent
  (GCC merges vbptr+vfptr -> 16, MSVC keeps them separate -> 32) #246
- static README: new section on internal linkage / access restriction
  with anonymous-namespace comparison #142
- 10-day practice: full problem list table + fixed day links #136
- new design_pattern/singleton/singleton_default.hpp: reusable
  thread-safe CRTP singleton template, compile+concurrency tested #303

site_extras/cpp-concurrency-false-sharing-cache-line.md

@@ -0,0 +1,54 @@
+> 来自 [C++那些事网站](https://light-city.github.io/stories_things/cpp-concurrency/false-sharing-cache-line/) 的文章，由站点同步脚本导出。
+
+# C++ False Sharing 与缓存行
+
+最近看到一段代码，手动做的对齐，于是研究一下不对齐又会带来什么影响？
+
+```cpp
+template <typename T>
+class AtomicWithPadding {
+ private:
+  static constexpr int kCacheLineSize = 64;
+  uint8_t padding_before[kCacheLineSize];
+
+ public:
+  std::atomic<T> value;
+
+ private:
+  uint8_t padding_after[kCacheLineSize];
+};
+```
+
+这里便引出了大家面试的必考点：
+
+- Cache line
+
+于是有下面几个面试问题，你能答上来？
+
+- 什么是cache line?
+- 什么是MESI协议？
+- 什么是False sharing?
+- C++里面如何保证避免False sharing?
+- 避免False sharing可以带来什么好处？
+- 你知道C++17之后提供了哪两个变量解决这个问题？
+
+于是，找了个性能测试，大概实现是：
+
+- 定义4个变量，a、b、c、d，多线程进行共享
+- 另外是对这四个变量对齐，然后开n个线程去调度。
+
+下面是benchmark：
+
+```cpp
+------------------------------------------------------------------
+Benchmark                        Time             CPU   Iterations
+------------------------------------------------------------------
+singleThread                  4.05 ms         4.04 ms          133
+directSharing/real_time       8.78 ms        0.069 ms           77
+falseSharing/real_time        8.81 ms        0.083 ms           74
+noSharing/real_time           1.16 ms        0.071 ms          578
+```
+
+可以看到noSharing得到了非常好的效果！缓存命中率大大提高，当然这里用perf分析一下是更好的，可以看看L1 cache miss。
+
+以上答案在知识星球已更新，欢迎一起探讨，留言区交流，谢谢~