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-projects-cpp-short-url-demo.md

@@ -0,0 +1,70 @@
+> 来自 [C++那些事网站](https://light-city.github.io/stories_things/cpp-projects/cpp-short-url-demo/) 的文章，由站点同步脚本导出。
+
+# C++ 短链接服务 Demo
+
+真实的短链接相对来说比较复杂，例如：hash算法、放号系统等等。
+
+今天只是从小项目角度模拟一个短链接实现。
+
+如何通过短短的几十行代码快速实现一个？其中涉及的几个关键点：
+
+- 如何将长链接缩短
+- 如何存储映射关系
+
+映射关系就不用说了，整一个map持久化内存，如果涉及到数据库，需要通过数据库的jdbc/odbc等接口访问写入库当中。
+
+长链接缩短这里采用随机算法+固定长度思路来实现。
+
+## 1.随机
+
+通过uniform_int_distribution，生成一个`[a, b]`区间的随机数，这里随机从下列字符串中每次取一个字符，最后拼接起来就是短链接。
+
+```cpp
+string characters{
+      "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"};
+```
+
+包含：
+
+- 数字
+- 小写字母
+- 大写字母
+
+```cpp
+uniform_int_distribution<int> distribution(0, characters.size() - 1);
+string code;
+for (int i = 0; i < length; ++i)
+  code += characters[distribution(generator)];
+```
+
+## 2.存储
+
+每次将其存储到map中即可，例如：
+
+```cpp
+if (links.count(originalLink)) {
+  return links[originalLink];
+}
+
+shortCode = generateRandomCode(len);
+string shortenedLink = base_url + shortCode;
+links[shortenedLink] = originalLink;
+```
+
+这里保留了基本url，例如：
+
+```cpp
+string baseUrl = "https://www.google.com.hk/search";
+LinkShortener shortener(baseUrl);
+
+string originalLink = "https://www.google.com.hk/search?q=asdsad&oq=asdsad&aqs=chrome.0.69i59j69i57j69i59l2.634j0j4&sourceid=chrome&ie=UTF-8";
+string shortenedLink = shortener.shortenLink(originalLink, 8);
+```
+
+最后便可以将这个url缩短为：
+
+```
+https://www.google.com.hk/searchtXuOlOSt
+```
+
+是不是很简单，本节的测试与完整代码已在星球更新，欢迎加入一起学习成长～