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-tools-bazel-debugging.md

@@ -0,0 +1,36 @@
+> 来自 [C++那些事网站](https://light-city.github.io/stories_things/cpp-tools/bazel-debugging/) 的文章，由站点同步脚本导出。
+
+# Bazel 与 LLDB 联合调试
+
+今天在调试时发现通过bazel编译出来的文件在mac上没法调试了，竟然全是汇编代码。
+
+```
+* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
+    frame #0: 0x0000000100000fd0 hello_world`main
+hello_world`main:
+->  0x100000fd0 <+0>: pushq  %rbp
+    0x100000fd1 <+1>: movq   %rsp, %rbp
+    0x100000fd4 <+4>: subq   $0x60, %rsp
+    0x100000fd8 <+8>: movl   $0x0, -0x4(%rbp)
+(lldb) quit
+```
+
+如何解决呢？
+
+默认是standalone，我们需要disable，这里采用local，这意味着输出将与其依赖项链接，而不是采用默认的`standalone`策略。
+
+```cpp
+build --spawn_strategy=local
+```
+
+这样的话可以看到代码了。
+
+```cpp
+* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
+    frame #0: 0x0000000100000fdf hello_world`main at hello_world.cc:14:21
+   11  
+   12   int main() {
+   13     // Create a thread pool
+-> 14     auto threadPool = GetCpuThreadPool();
+   15     if (!threadPool) {
+```