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/stl-source-analysis-cpp-string-interview.md

@@ -0,0 +1,33 @@
+> 来自 [C++那些事网站](https://light-city.github.io/stories_things/stl-source-analysis/cpp-string-interview/) 的文章，由站点同步脚本导出。
+
+# STL string 面试题精讲
+
+今天给大家分享一个字符串相关的面试题目，总共有4个case，可以在留言区评论，说出这四处会发生什么。
+
+std::string大家应该非常熟悉了，std::string_view是17之后引入的视图，不懂的可以查阅一下，下面有两个字面量，分别是string子面量与string_view字面量，用`s`、`sv`代表。
+
+前两个问题都是函数返回一个std::string_view，但有些区别，一个是基于c-style string，另一个基于string。
+
+后面两个问题是从string字面量与string_view字面量构建。
+
+好了，问题描述清楚了，可以在留言区见了～
+
+```cpp
+std::string_view getBar() {
+  std::string b{"it is bar."};
+  return b;
+}
+
+std::string_view getFoo() {
+  return "It is foo.";
+}
+
+using namespace std::string_literals;
+using namespace std::string_view_literals;
+std::cout << getFoo() << std::endl;   // 1
+std::cout << getBar() << std::endl;	  // 2
+std::string_view svf{"Hello foo"s};  
+std::cout << svf << std::endl;  			// 3
+std::string_view svff{"Hello foo"sv}; 
+std::cout << svff << std::endl; 			// 4
+```