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

tool/dbg-macro/README.md

@@ -0,0 +1,70 @@
+> 本文档由 [C++那些事网站](https://light-city.github.io/stories_things/cpp-tools/dbg-macro/) 同步生成，网站版本含多语言与更新排版。
+
+# C++ dbg-macro 调试宏
+
+打日志是 C++ 开发中必不可少的一种 debug 方式，dbg-macro 受 Rust 语言中 的 `dbg` 启发，提供比 `printf` 和 `std::cout` 更好的宏函数。主要有如下特点：
+
+- 美观的彩色输出（当输出不是交互式终端时，颜色将自动禁用）
+- 兼容 C++11，并且是 header-only
+- 支持基础类型和 STL 容器类型的输出
+- 可以输出文件名、行号、函数名和原始表达式
+
+例如下面的代码：
+
+```cpp
+#include <vector>
+#include <dbg.h>
+
+// You can use "dbg(..)" in expressions:
+int factorial(int n) {
+  if (dbg(n <= 1)) {
+    return dbg(1);
+  } else {
+    return dbg(n * factorial(n - 1));
+  }
+}
+
+int main() {
+  std::string message = "hello";
+  dbg(message);  // [example.cpp:15 (main)] message = "hello" (std::string)
+
+  const int a = 2;
+  const int b = dbg(3 * a) + 1;  // [example.cpp:18 (main)] 3 * a = 6 (int)
+
+  std::vector<int> numbers{b, 13, 42};
+  dbg(numbers);  // [example.cpp:21 (main)] numbers = {7, 13, 42} (size: 3) (std::vector<int>)
+
+  dbg("this line is executed");  // [example.cpp:23 (main)] this line is executed
+
+  factorial(4);
+
+  return 0;
+}
+```
+
+效果如下：
+
+![](https://camo.githubusercontent.com/3cde47c8db560b3ed42763f2ef306d78d4b19087/68747470733a2f2f692e696d6775722e636f6d2f4e4845596b39412e706e67)
+
+## 安装
+
+```bash
+git clone https://github.com/sharkdp/dbg-macro
+sudo ln -s $(readlink -f dbg-macro/dbg.h) /usr/include/dbg.h
+```
+
+这样，你就可以在任何地方使用 `dbg.h` 了。
+
+### 在 Arch Linux 上
+
+```bash
+yay -S dbg-macro
+```
+
+### 使用 vcpkg
+
+```bash
+vcpkg install dbg-macro
+```
+
+更多内容详见 [官方文档](https://github.com/sharkdp/dbg-macro)。