feat(runtime): cancel & future combinator

[George-Miao]

Closes #604

[Copilot]

Pull request overview

This pull request adds cancellation support to the compio runtime through a new CancelToken API and future combinators. The implementation allows multiple operations to be cancelled together using a token, with both "fail-fast" and "fail-slow" cancellation modes.

Changes:

• Introduces CancelToken for cancelling multiple operations at once
• Adds with_cancel future combinator (requires future-combinator feature)
• Refactors driver-level cancel API to accept references instead of owned values
• Adds Runtime::current() and Runtime::try_current() helper methods

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
compio-runtime/src/cancel.rs	New file implementing CancelToken and WaitFuture for coordinated cancellation
compio-runtime/src/future/cancel.rs	New file implementing WithCancel and WithCancelFailFast future wrappers
compio-runtime/src/future/mod.rs	Extends FutureExt trait with with_cancel method and adds cancel field to Ext
compio-runtime/src/runtime/future.rs	Adds ContextExt trait and integrates cancel token registration in Submit::poll
compio-runtime/src/runtime/mod.rs	Adds current(), try_current() methods and internal cancel token management methods
compio-runtime/src/lib.rs	Exports CancelToken and conditionally exports future module
compio-driver/src/lib.rs	Changes register_cancel to accept &Key<T> instead of Key<T>
compio-driver/src/cancel.rs	Adds cancels method and optimizes duplicate registration handling
compio-driver/tests/file.rs	Updates test to pass reference to register_cancel
compio-runtime/Cargo.toml	Adds synchrony dependency for event handling

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.