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-build-guide.md

@@ -0,0 +1,61 @@
+> 来自 [C++那些事网站](https://light-city.github.io/stories_things/cpp-tools/bazel-build-guide/) 的文章，由站点同步脚本导出。
+
+# Bazel 构建 C++ 项目
+
+```
+gpadmin@gpadmin:~$ ./bazel-6.1.1-installer-linux-x86_64.sh --user
+Bazel installer
+---------------
+
+Bazel is bundled with software licensed under the GPLv2 with Classpath exception.
+You can find the sources next to the installer on our release page:
+   https://github.com/bazelbuild/bazel/releases
+
+# 
+
+## Build information
+   - [Commit](https://github.com/bazelbuild/bazel/commit/1a90fe5)
+Uncompressing......WARNING: Invoking Bazel in batch mode since it is not invoked from within a workspace (below a directory having a WORKSPACE file).
+Extracting Bazel installation...
+.
+
+Bazel is now installed!
+
+Make sure you have "/home/gpadmin/bin" in your path.
+
+For bash completion, add the following line to your :
+  source /home/gpadmin/.bazel/bin/bazel-complete.bash
+
+For fish shell completion, link this file into your
+/home/gpadmin/.config/fish/completions/ directory:
+  ln -s /home/gpadmin/.bazel/bin/bazel.fish /home/gpadmin/.config/fish/completions/bazel.fish
+
+See http://bazel.build/docs/getting-started.html to start a new project!
+```
+
+
+
+默认安装到home/bin
+
+
+
+```
+gpadmin@gpadmin:~$ ls ~/bin/
+bazel
+```
+
+
+
+
+
+1. 在安装过程中，Bazel 可能会提示您添加环境变量。如果是这样，请根据提示编辑您的 `~/.bashrc` 文件并将以下行添加到文件末尾：
+
+```
+复制代码export PATH="$PATH:$HOME/bin"
+```
+
+1. 重新加载 `.bashrc` 文件以使更改生效：
+
+```
+复制代码source ~/.bashrc
+```