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-idioms-header-only-vs-implementation.md

@@ -0,0 +1,11 @@
+> 来自 [C++那些事网站](https://light-city.github.io/stories_things/cpp-idioms/header-only-vs-implementation/) 的文章，由站点同步脚本导出。
+
+# C++ Header-Only 与实现分离
+
+最近看到一些小伙伴问到了几个比较有趣的问题，这里总结一下，这些都是实际面试中出现过的问题，看看你知道多少，考察一下底子。
+
+**面试问题1：你通常编写代码的风格是什么？Header-Only？还是声明与实现分离？**
+
+**面试问题2：C++里面是否所有的函数可以分离实现与声明？为什么？**
+
+**面试问题3：模版的声明与实现分离有哪些方法？请举例说明各自优缺点，适用场景？**