Print nextest-style cancellation banner on Ctrl+C

[MatthewMckee4]

Summary

Pressing Ctrl+C during a test run used to terminate karva with no
indication of what was cut short. The orchestrator now mirrors nextest:
when a SIGINT is received it prints Cancelling due to interrupt: N tests still running and, before killing each remaining worker, emits a
SIGINT [duration] worker N (M tests) line in the same column layout as
the PASS/FAIL lines. The cancellation banner is only emitted on
genuine interrupts; fail-fast shutdowns continue to be silent at the
orchestrator level. The Ctrl+C handler is now installed at the top of
run_parallel_tests rather than after worker spawn so an early SIGINT
during collection or partitioning no longer slips through to the default
disposition.

Sample output:

    Starting 13 tests across 3 workers
  Cancelling due to interrupt: 13 tests still running
      SIGINT [   2.970s] worker 0 (5 tests)
      SIGINT [   2.970s] worker 1 (4 tests)
      SIGINT [   2.969s] worker 2 (4 tests)

The reported test count is per-worker (each worker's assigned partition
size). Workers only flush results to the cache on exit, so the
orchestrator can't see which individual tests have already completed
inside a still-running worker; the count is therefore an upper bound on
tests still in flight.

Test Plan

New unix-only integration test in crates/karva/tests/it/cancel.rs
spawns karva on a 60s-sleeping test, sends SIGINT via kill -s INT,
and asserts both the cancellation banner and a SIGINT line appear in
stdout. just test and uvx prek run -a pass.

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 1 untouched benchmark

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_{Comparing nextest-style-cancellation (69cd108) with main (eb3c800)}