fix(runtime): split the runtime to avoid ref cycle

[Berrysoft]

Closes #909

I think this solution is better. Let's see the ASan results.

cc: @paddor

[Copilot]

Pull request overview

This PR refactors compio-runtime to break a runtime/task reference cycle by splitting Runtime into separately reference-counted components and updating cancellation/submission/timer futures to hold driver/timer handles rather than a full Runtime clone. It also adds a regression test asserting that detached tasks are dropped when the runtime is dropped, even if they hold a CancelToken.

Changes:

• Refactor Runtime to hold Rc<Executor> and Rc<RefCell<Proactor>> (and timer runtime) directly, with new current_driver() / current_timer_runtime() helpers.
• Update CancelToken, Submit, SubmitMulti, and timer futures to use the shared driver/timer runtime handles and move driver polling helpers into future/mod.rs.
• Add a new drop regression test to detect potential Rc cycles keeping tasks alive across runtime drop.

Reviewed changes

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

Show a summary per file

File	Description
compio-runtime/tests/drop.rs	Adds a regression test ensuring a detached task holding a CancelToken is dropped when the runtime drops.
compio-runtime/src/time.rs	Moves timer creation logic into time.rs and makes timer futures own an Rc<RefCell<TimerRuntime>>; adds TimerRuntime::poll_timer.
compio-runtime/src/lib.rs	Splits Runtime into separate Rc fields, adjusts submit APIs to pass driver handles, and changes drop logic to key off executor refcount.
compio-runtime/src/future/stream.rs	Changes SubmitMulti to store the driver handle and use shared poll/submit helpers.
compio-runtime/src/future/mod.rs	Introduces shared internal helper functions for submit/poll operations against Proactor.
compio-runtime/src/future/future.rs	Changes Submit to store the driver handle and use shared poll/submit helpers.
compio-runtime/src/cancel.rs	Changes CancelToken internals to store the driver handle (instead of Runtime) and adjusts debug output accordingly.

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