refactor, add C++17 fallback for threaded_pipeline_t via doorbell abstraction

[steven-varga]

Context

threaded_pipeline_t (#197) and H5Qall.hpp (#195) require C++20. Three primitives drive that requirement:

Primitive	Header	C++17 substitute
std::bit_ceil	<bit>	3-line constexpr helper — trivial
std::stop_token / std::stop_source	<stop_token>	std::atomic<bool> flag — easy
std::jthread	<thread>	std::thread + RAII dtor wrapper — easy
std::atomic::wait() / notify_one() / notify_all()	<atomic>	std::mutex + std::condition_variable — meaningful

The atomic wait/notify maps directly to OS futex primitives (Linux futex, Windows WaitOnAddress, macOS ulock_wait) with no mutex — that is what makes the queues lock-free on the contention path. The condvar replacement reintroduces a mutex per wait/notify call, breaking lock-freedom.

For h5cpp's actual workload (large chunks, CPU-bound compression, H5Dwrite_chunk serialized to the main thread) the condvar overhead is below the noise floor. Correctness is fully preserved.

Approach

Do not modify H5Qall.hpp — it is a self-contained MIT-licensed component and stays C++20-pure.

Introduce h5cpp/detail/doorbell.hpp selected at compile time via the C++20 feature-test macro:

#ifdef __cpp_lib_atomic_wait
    using doorbell_t = atomic_doorbell_t;   // lock-free, C++20
#else
    using doorbell_t = condvar_doorbell_t;  // mutex+condvar, C++17
#endif

H5Zpipeline_threaded.hpp builds its internal queues on doorbell_t instead of directly on std::atomic. The C++20 path is unchanged. The C++17 path is correct and requires no downstream changes.

std::jthread and std::stop_token in H5Zpipeline_threaded.hpp get the same treatment via a thin detail::stoppable_thread_t wrapper over std::thread + std::atomic<bool>.

Constraints

• H5Qall.hpp must remain C++20-only — do not add #ifdef guards inside it.
• The C++20 code path must be bit-for-bit identical to the current implementation.
• Two commits: (1) detail/doorbell.hpp + detail/stoppable_thread.hpp abstractions, (2) wire H5Zpipeline_threaded.hpp onto them and drop the hard #error guard.
• C++17 minimum: GCC 9, Clang 9, MSVC 19.14 (VS 2017 15.7).

Motivation

Targets HPC environments and older toolchains that mandate C++17. Decision on priority pending outcome of HDFGroup platform discussion (Gerd Heber, 2026-05-23).