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/modern-cpp-cpp11-std-bind-vs-lambda.md

@@ -0,0 +1,58 @@
+> 来自 [C++那些事网站](https://light-city.github.io/stories_things/modern-cpp/cpp11/std-bind-vs-lambda/) 的文章，由站点同步脚本导出。
+
+# C++ std::bind 与 lambda 对比
+
+https://quick-bench.com/q/CEnR_XUuIwnZxaP4KJNSlpYG38c
+
+```
+#include <functional>
+#include <numeric>
+#include <vector>
+
+
+
+struct Calculator {
+    int count{1};
+    int total{0};
+
+    auto calculate() -> void {
+        using std::iota, std::vector;
+
+        // Random math
+        vector<int> u(8), v(Calculator::count * u.size());
+
+        iota(v.begin(), v.end(), *u.begin());
+
+        int sum{*v.begin() + *u.begin()};
+
+        for (size_t pos{0}; pos < v.size(); ++pos) {
+            sum += u[pos % u.size()] * v[pos];
+        }
+
+        Calculator::total += sum;
+        ++Calculator::count;
+    }
+};
+
+// auto f(std::function<void(int)> g) -> void { g(rand()); }
+
+template<typename FN> auto f(FN g) -> void { g(rand()); }
+
+
+static auto BM_bind(benchmark::State& state) -> void {
+    Calculator calculator;
+    for (auto _ : state) {
+        f(std::bind(&Calculator::calculate, &calculator));
+    }
+}
+
+static auto BM_lambda(benchmark::State& state) -> void {
+    Calculator calculator;
+    for (auto _ : state) {
+        f([&calculator]([[maybe_unused]] int x) { calculator.calculate(); });
+    }
+}
+
+BENCHMARK(BM_bind);
+BENCHMARK(BM_lambda);
+```