fix: add cancel function to the Timer API to make it behave correctly with finalizers and selectables

[algebraic-dev]

This PR aims to fix the Timer API selector to make it finish as soon as possible when unregistered. This change makes the Selectable.one function drop the selectables array as soon as possible, so when combined with finalizers that have some effects like the TCP socket finalizer, it runs it as soon as possible.

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 5ef7b45afaedd794fb39b0fbd70924b573694a00 --onto 7d55c033e1f0ad8cb7c2b9ab72e5db40ce192222. You can force Mathlib CI using the force-mathlib-ci label. (2025-10-01 04:46:57)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 5ef7b45afaedd794fb39b0fbd70924b573694a00 --onto d88e417cda0f7afb1e15c806c6bfdbbb09ba7eef. You can force Mathlib CI using the force-mathlib-ci label. (2025-10-02 00:15:42)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 5ef7b45afaedd794fb39b0fbd70924b573694a00 --onto 0b2193c7715a55246d0596a4db548c1b5b81562a. You can force Mathlib CI using the force-mathlib-ci label. (2025-10-03 15:31:53)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 5ef7b45afaedd794fb39b0fbd70924b573694a00 --onto 54c6efea95a0d478f2659632749035962b1f4d8d. You can force Mathlib CI using the force-mathlib-ci label. (2025-10-07 21:37:21)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 5ef7b45afaedd794fb39b0fbd70924b573694a00 --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-10-01 04:46:58)

[TwoFX]

This clearly needs tests!