Fix C++17 object lifetime handling in type-erasure storage

[vshalygin]

This issue was identified during a Codex code review.

cl::function and the external-storage branch of value_proxy write pointer objects into std::byte buffers through reinterpret_cast without explicitly starting the lifetime of those pointer objects.

Although this works with the currently supported compilers, it is not formally well-defined under the project's minimum C++17 standard.

Use placement new and proper typed access, or replace the raw byte-storage representation with an explicitly managed pointer/union representation.