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

design_pattern/singleton/singleton_default.hpp

@@ -0,0 +1,58 @@
+/**
+ * @file singleton_default.hpp
+ * @brief 可复用的线程安全单例模板基类（CRTP）
+ * 受 issue #303 启发：https://github.com/Light-City/CPlusPlusThings/issues/303
+ * 参考 boost::container::singleton_default 的思路。
+ *
+ * 用法：
+ *   class MyConfig : public SingletonDefault<MyConfig> {
+ *       friend class SingletonDefault<MyConfig>;
+ *   private:
+ *       MyConfig() = default;          // 构造私有，交由基类创建
+ *   public:
+ *       int value = 42;
+ *   };
+ *   MyConfig::instance().value = 1;
+ *
+ * 编译：g++ -std=c++11 main.cpp
+ * @author 光城
+ * @version v1
+ * @date 2026-10
+ */
+
+#pragma once
+
+#include <mutex>
+
+template <class T>
+class SingletonDefault {
+public:
+    SingletonDefault(const SingletonDefault&) = delete;
+    SingletonDefault& operator=(const SingletonDefault&) = delete;
+
+    static T& instance() {
+        // C++11 起，局部静态变量的初始化由标准保证线程安全（"magic statics"）
+        static T inst;
+        return inst;
+    }
+
+protected:
+    SingletonDefault() = default;
+    ~SingletonDefault() = default;
+};
+
+/**
+ * 若需要 C++03 或对 magic statics 不放心的环境，可用 call_once 版本：
+ *
+ * template <class T>
+ * class SingletonDefaultOnce {
+ * public:
+ *     static T& instance() {
+ *         std::call_once(once_, [] { inst_.reset(new T); });
+ *         return *inst_;
+ *     }
+ * private:
+ *     static std::once_flag once_;
+ *     static std::unique_ptr<T> inst_;
+ * };
+ */