Unify event implementation

[msimberg]

This is on top of #199.

Uses the explicit implementation of a cudaEvent_t RAII wrapper in include/ghex.device/cuda/event.hpp in the device future implementation.

[Copilot]

Pull request overview

This PR unifies CUDA event usage by reusing the explicit cuda_event RAII wrapper (include/ghex/device/cuda/event.hpp) inside the CUDA future implementation, and aligns operator bool() semantics for CUDA stream/event with PR #199.

Changes:

• Invert operator bool() for ghex::device::stream and ghex::device::cuda_event to return true when the object is valid (not moved-from).
• Replace future’s internal cudaEvent_t managed-struct wrapper with the shared cuda_event RAII type.
• Update cuda_event to accept event creation flags and add implicit conversions to cudaEvent_t references; adjust event_pool assertion accordingly.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
include/ghex/device/cuda/stream.hpp	Aligns operator bool() validity semantics with the new convention.
include/ghex/device/cuda/future.hpp	Switches future’s event storage to cuda_event (but introduces an include-guard issue).
include/ghex/device/cuda/event_pool.hpp	Updates assertion to match the inverted cuda_event::operator bool() semantics.
include/ghex/device/cuda/event.hpp	Adds flag-configurable ctor and aligns operator bool() validity semantics; adds implicit cudaEvent_t conversions.

Comments suppressed due to low confidence (1)

include/ghex/device/cuda/future.hpp:17

• future.hpp now includes ghex/device/cuda/event.hpp unconditionally. That header depends on CUDA runtime types/macros and is not guarded by GHEX_CUDACC, so this will break any translation unit that includes future.hpp without GHEX_CUDACC (e.g., generic headers like include/ghex/packer.hpp include this file unconditionally). Consider moving the CUDA-only includes (device/cuda/event.hpp, and any other CUDA-runtime-dependent headers) inside the #ifdef GHEX_CUDACC block, or include the wrapper header ghex/device/event.hpp instead so non-CUDA builds see the stub type.

#include <ghex/config.hpp>
#include <ghex/device/cuda/event.hpp>
#include <ghex/device/cuda/error.hpp>
#ifdef GHEX_CUDACC
#include <ghex/device/cuda/runtime.hpp>
#endif

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[philip-paul-mueller]

I have some comments/questions.

I kind of think that the number of places where we have amended the meaning is a bit low?