Modernize copy prevention in basic_otstream and TracerImpl (#674)
- `basic_otstream`: replace private-undefined copy ctor with explicit `=
delete` for both copy ctor and copy assignment in the public section.
Adds a one-liner comment explaining the destructor side-effect (`delete
this->rdbuf()` causes double-free if two objects share the same buffer).
- `TracerImpl`: move the already-`= delete`d copy ctor from private
section to public, and add the missing copy assignment `= delete`.
Part of the ongoing Rule-of-Zero / modern C++ idiom cleanup in `basic/`.