std::pmr 与 Polymorphic Allocator | STL

本章覆盖：`<memory_resource>`；`std::pmr`；`polymorphic_allocator`；runtime memory resource；pmr container alias；memory resource lifetime；decouple container type from concrete resource；C++17；high-throughput / temporary arena 场景；复杂度来自生命周期与所有权，不是免费优化。