…ndency

Use Promise/Future to control completion instead of futures::sleep,
eliminating wall-clock timing dependencies that cause flaky failures
on loaded CI systems.

Remove timing assertions (T_CHECK_TIMEOUT) that cause flakiness on
Windows due to coarse timer resolution (~15.6ms). The test now verifies
only the cancellation logic: which callbacks fired and which were
cancelled. Timeout values increased to 50/100/200ms to be well above
Windows timer resolution.