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-class-template-over-function-template.md

@@ -0,0 +1,35 @@
+> 来自 [C++那些事网站](https://light-city.github.io/stories_things/cpp-idioms/class-template-over-function-template/) 的文章，由站点同步脚本导出。
+
+# C++ 类模板优于函数模板
+
+https://stackoverflow.com/questions/5688355/partial-specialisation-of-member-function-with-non-type-parameter
+
+最近看到一个很有意思的模版问题，看看大家对函数模版与类模版的基础掌握的如何，对于下面这个例子会出现什么问题？
+
+你一般使用什么方法进行修复？
+
+```cpp
+template <typename T, int R>
+struct foo {
+  foo(const T& v) : value_(v) {}
+
+  void bar() {
+    std::cout << "Generic" << std::endl;
+    for (int i = 0; i < R; ++i) std::cout << value_ << std::endl;
+  }
+
+  T value_;
+};
+
+template <>
+void foo<float, 3>::bar() {
+  std::cout << "Float" << std::endl;
+  for (int i = 0; i < 3; ++i) std::cout << value_ << std::endl;
+}
+
+template <int R>
+void foo<double, R>::bar() {
+  std::cout << "Double" << std::endl;
+  for (int i = 0; i < R; ++i) std::cout << value_ << std::endl;
+}
+```