Modernizing HPX: Architectural stdexec Migration and Subsystem Integration

This project completes the architectural transition of the HPX parallel runtime to the C++26 Sender/Receiver (S/R) model. Building upon the recent cleanup of legacy execution code, I will advance HPX from an "adapter-based" architecture to its "Native Architectural Phase." This involves refactoring the core execution boundaries of make_future and keep_future to remove internal implementation dependencies (such as the __loop_ pointer) and porting distributed subsystems like MPI and CUDA to be native, first-class stdexec citizens. My primary objective is to eliminate technical debt and achieve zero-overhead integration with the C++26 execution standard while ensuring full backwards compatibility for all existing HPX applications.