Problem: switching contests while a run, interactive session, or stress
test was in progress left orphaned callbacks and terminal jobs alive.
Running `:CP run` twice also allowed the first run's stale output to
overwrite the buffer after the second run completed.
Solution: replace the `io_view_running` bool in `views.lua` with a
generation counter (`_run_gen`); each `run_io_view` call increments it
and captures a local `gen`, and async callbacks bail out early when `gen
~= _run_gen`. Add `cancel_io_view`, `cancel_interactive` to `views.lua`
and `cancel` to `stress.lua` for forceful teardown without layout
restore. Call these in `setup_contest` whenever `is_new_contest` is true.
Barrett Ruth
9f23076420