fix(rsp): cancel arena compare background thread when TUI exits

[elliottneilclark]

run_comparison_with_tui spawned the comparison thread, dropped the
JoinHandle, and swallowed tx.send errors inside the callback — so
after the user quit the TUI the comparison kept running, allocating a
full CFR tree per remaining permutation, and the temp output directory
could be dropped out from under the OHH historian.

Add ArenaComparison::run_with_cancellable_callback that takes a
closure returning ControlFlow<()> and short-circuits both the
permutation loop and the outer game loop on Break. Keep the existing
run_with_callback as a Continue-always convenience wrapper.

Share an Arc<AtomicBool> between the TUI-side runner and the worker
callback; the callback flips it on send failure and returns Break
so the worker stops as soon as the current permutation ends. Keep the
worker's JoinHandle, set the flag once the TUI event loop returns,
and join() the handle before returning — that keeps any caller-owned
temp directory alive until the OHH historian is done writing.

Add a regression test that run_with_cancellable_callback stops after
~3 permutations instead of running all 100 configured games.