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-stl-debug-mode.md

@@ -0,0 +1,80 @@
+> 来自 [C++那些事网站](https://light-city.github.io/stories_things/cpp-tools/stl-debug-mode/) 的文章，由站点同步脚本导出。
+
+# C++ STL Debug 模式
+
+## 问题
+
+Project阶段的schema里面的fields发生了变化，fields是一个vector节点，每个成员是`std::shared_ptr<arrow::Field>`，对比前后的vector指针发现`_M_impl`的`_M_start`位置发生了变化，因此需要gdb watch其变化，查看何处改了它。
+
+## Vector结构
+
+vecotr代码：
+
+> https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/bits/stl_vector.h
+
+```cpp
+typedef typename __gnu_cxx::__alloc_traits<_Tp_alloc_type>::pointer
+       	pointer;
+struct _Vector_impl_data
+{
+	pointer _M_start;
+	pointer _M_finish;
+	pointer _M_end_of_storage;
+}
+```
+
+需要知道几个重要信息：
+
+- `_M_start`与`_M_finish`等都是pointer类型
+- pointer类型是原始指针类型
+
+有了这个信息，我们watch便可以非常方便的观察这个`_M_start`发生了什么变化，注意：是这个指针`_M_start`发生了变化，而不是其值发生了变化。
+
+## Watch
+
+首先打印其地址:
+
+```cpp
+(gdb) p &((arrow::Schema::Impl *)this->output_schema_->impl_)->fields_._M_impl._M_start
+$10 = (std::_Vector_base<std::shared_ptr<arrow::Field>, std::allocator<std::shared_ptr<arrow::Field> > >::pointer *) 0xb11dd0
+```
+
+我们得到了`pointer* `的地址，然后watch它。
+
+```cpp
+(gdb) watch *((std::_Vector_base<std::shared_ptr<arrow::Field>, std::allocator<std::shared_ptr<arrow::Field> > >::pointer *) 0xb11dd0)
+Hardware watchpoint 2: *((std::_Vector_base<std::shared_ptr<arrow::Field>, std::allocator<std::shared_ptr<arrow::Field> > >::pointer *) 0xb11dd0)
+```
+
+随后我们continue，变得了变更点：
+
+```cpp
+(gdb) c
+Continuing.
+
+
+Thread 1 "postgres" hit Hardware watchpoint 2: *((std::_Vector_base<std::shared_ptr<arrow::Field>, std::allocator<std::shared_ptr<arrow::Field> > >::pointer *) 0xb11dd0)
+
+Old value = (std::_Vector_base<std::shared_ptr<arrow::Field>, std::allocator<std::shared_ptr<arrow::Field> > >::pointer) 0x979470
+New value = (std::_Vector_base<std::shared_ptr<arrow::Field>, std::allocator<std::shared_ptr<arrow::Field> > >::pointer) 0x9fe1e0
+VecSeqNext (node=0x9fdc40) at src/backend/vecexecutor/nodeSeqscan.c:210
+210		if (table_scan_getnextslot(scandesc, direction, slot))
+(gdb) bt
+#0  VecSeqNext (node=0x9fdc40) at src/backend/vecexecutor/nodeSeqscan.c:210
+#1  0x00007f45994e5c70 in ExecVecScanFetchInternal
+    (node=0x9fdc40, accessMtd=0x7f45994e6335 <VecSeqNext>, recheckMtd=0x7f45994e6430 <VecSeqRecheck>) at src/backend/vecexecutor/execScan.c:146
+#2  0x00007f45994e58da in ExecVecScanFetch (node=0x9fdc40, accessMtd=0x7f45994e6335 <VecSeqNext>, recheckMtd=0x7f45994e6430 <VecSeqRecheck>)
+    at src/backend/vecexecutor/execScan.c:40
+#3  0x00007f45994e5d0b in ExecVecScan
+    (node=0x9fdc40, vestate=0x9fdd48, accessMtd=0x7f45994e6335 <VecSeqNext>, recheckMtd=0x7f45994e6430 <VecSeqRecheck>)
+    at src/backend/vecexecutor/execScan.c:200
+#4  0x00007f45994e6487 in ExecVecSeqScan (pstate=0x9fdc40) at src/backend/vecexecutor/nodeSeqscan.c:246
+#5  0x00007f45994c40ce in VecExecProcNodeGPDB (node=0x9fdc40) at src/backend/hook/executor.c:102
+#6  0x00007f45994c41c2 in VecExecProcNodeFirst (node=0x9fdc40) at src/backend/hook/executor.c:153
+#7  0x00007f45994dc610 in ExecProcNode (node=0x9fdc40) at /home/gpadmin/install/cbdb/include/postgresql/server/executor/executor.h:281
+#8  0x00007f45994e2065 in get_current_next_batch (node=0x9fdc40) at src/backend/vecexecutor/execMain.c:1851
+#9  0x00007f4599375c76 in get_arrow_record_batch(GetNextCallback, void*) (fn=0x7f45994e204d <get_current_next_batch>, arg=0x9fdc40)
+    at c_glib/arrow-glib/reader.cpp:216
+
+
+```