support async futex

[wokron]

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[Copilot]

Pull request overview

This PR introduces a coroutine-friendly futex primitive (condy::Futex) and adds corresponding tests and an example demonstrating a producer/consumer queue built on top of it.

Changes:

• Add include/condy/futex.hpp implementing condy::Futex<T> with async wait() and thread-safe notify_one/notify_all.
• Add doctest coverage for async futex behaviors (basic notify, notify-all, cross-thread wake, cancellation, and a queue built from futex-based primitives).
• Add a new queue example and document/build it; export futex.hpp from condy.hpp.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
include/condy/futex.hpp	New futex implementation and sender-based wait API.
tests/test_futex.cpp	New doctest suite validating condy::Futex behavior and showcasing a futex-based queue.
include/condy.hpp	Exports condy/futex.hpp from the umbrella header.
examples/queue.cpp	Adds a futex-based producer/consumer queue example using two runtimes.
examples/CMakeLists.txt	Builds the new queue example; fixes formatting for futex-semaphore link line.
docs/examples.md	Documents the new queue.cpp example.

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[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

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