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-memory-debug-valgrind-massif-memory.md

@@ -0,0 +1,19 @@
+> 来自 [C++那些事网站](https://light-city.github.io/stories_things/cpp-memory-debug/valgrind-massif-memory/) 的文章，由站点同步脚本导出。
+
+# Valgrind Massif 内存分析
+
+gpstart -a --wrapper=valgrind --wrapper-args=" --tool=massif --time-unit=B --detailed-freq=1 --massif-out-file=/tmp/massif.out.%p"
+
+![](../assets/images/2024/mem/massif/mss.png)
+
+```
+ms_print xxx.out | more
+```
+
+https://valgrind.org/docs/manual/ms-manual.html
+
+![](../assets/images/2024/mem/massif/ms_print.png)
+
+
+
+valgrind -v --tool=massif --time-unit=B --detailed-freq=1 --massif-out-file=./massif.out  ./a.out