refactor: remove virtual destructor from BasicIO to restore CRTP zero-overhead

[LHT129]

Background

BasicIO<IOTmpl> uses CRTP (Curiously Recurring Template Pattern) for static polymorphism. All *Impl methods are dispatched via static_cast<IOTmpl&>(*this) — there is no dynamic dispatch.

However, the destructor is declared as virtual ~BasicIO() = default; in src/io/basic_io.h:59. This is the only virtual function in the entire hierarchy.

Problem

The virtual destructor introduces an unnecessary vtable pointer (vptr, 8 bytes) into every IO object:

• CRTP is designed for zero-overhead static polymorphism — the vptr defeats this promise
• No IO object is ever deleted through a BasicIO<>* base pointer (there is no public virtual base class)
• The vptr wastes 8 bytes per object and adds an unnecessary indirect jump

Proposed Fix

1. Change virtual ~BasicIO() = default; to a protected non-virtual ~BasicIO() = default;

   • Removes the vptr, restoring zero overhead
   • protected access prevents external code from deleting through a base pointer (compile-time safety)

2. Remove override from all 7 subclass destructors:

   • MemoryIO, BufferIO, MMapIO, AsyncIO, ReaderIO, MemoryBlockIO, NonContinuousIO

Expected Impact

• -8 bytes per IO object (vptr elimination)
• Eliminates unnecessary virtual dispatch overhead
• Restores the CRTP zero-overhead design intent

[vsag-bot]

/kind improvement
/module io