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-trigraphs-cpp.md

@@ -0,0 +1,51 @@
+> 来自 [C++那些事网站](https://light-city.github.io/stories_things/cpp-idioms/trigraphs-cpp/) 的文章，由站点同步脚本导出。
+
+# C++ 三字母词
+
+## C++那些事之删除三字母
+
+C++17 之前，Trigraph 是用 ISO/IEC 646 等小字符代码编写程序的功能，在 C89 中引入。 ISO/IEC 646 缺少 C 中使用的以下字符，因此`??`请使用以下字符进行转义：
+
+| 三字母内容 | 替换的内容 |
+| ---------- | ---------- |
+| `??=`      | `#`        |
+| `??(`      | `[`        |
+| `??)`      | `]`        |
+| `??<`      | `{`        |
+| `??>`      | `}`        |
+| `??/`      | `\`        |
+| `??!`      | `|`        |
+| `??-`      | `~`        |
+| `??'`      | `^`        |
+
+`\?`它被转义以防止被三字母表替换。
+
+C++17三字母被删除，`??=`等不再被替换。
+
+## 示例
+
+C++17之前：
+
+```cpp
+std::cout << "??=" << std::endl;
+std::cout << "??(" << std::endl;
+std::cout << "?\?(" << std::endl;
+```
+
+输出：
+
+```
+#
+[
+??(
+```
+
+C++17及之后：
+
+```
+??=
+??(
+??(
+```
+
+现在可能由于Unicode等字符编码已经普遍存在，不再需要三字母组，而gcc等编译器默认禁用三字母组。