Typed access to the allocator of a shared memory segment.
The accessor is a small value type (one pointer) that keeps the boost interprocess headers out of EventBus.h: objects stored in shared memory hold a copy of the accessor, while the process local allocator it points to is owned by SharedMemoryService. Only the raw block allocation itself is untyped, everything above it stays typed.
Copying is intentional: the copy lives inside the shared memory object and therefore keeps a stable address for the whole lifetime of that object, independent of the process that created it.
