## Problem
Switching contests while a run, interactive session, or stress test was
active left orphaned callbacks and terminal jobs alive. Running `:CP
run`
twice also let the first run's stale output overwrite the buffer.
## Solution
Replace the `io_view_running` bool in `views.lua` with a generation
counter so concurrent `run_io_view` calls self-cancel via stale-gen
checks in async callbacks. Add `cancel_io_view`, `cancel_interactive`,
and `stress.cancel` for forceful teardown, and call them in
`setup_contest` whenever `is_new_contest` is true.
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