test(oneshot): race receiver completion against the sender wake

[orthur2]

Summary

Add a regression test for the oneshot reclamation race fixed in #361. It registers the receiver's waker first so that sending or dropping the sender always takes the waking path, then completes and drops the receiver while the sender may still be inside that path. A single race rarely lands in the window between the final state swap and the helper's return, so the test repeats it 128 times under Miri.

With asyncband/src/oneshot reverted to the code before #361, the existing oneshot_test passes all 16 seeds in 0..16 under the CI Miri setup, while this test fails at 6 of them, including seeds 0 and 1 that CI runs. With the fix it passes all 16 and adds about 6 seconds per seed under Miri.

[tisonkun]

Thank you!