feat: ensure teardown always runs (even when setup fails or on SIGINT/SIGTERM)

[andreasjansson]

Summary

This PR ensures that _teardown.txt always runs, regardless of:

• Whether tests passed or failed
• Whether setup failed (main tests are skipped but teardown still runs)
• Whether the process was interrupted by SIGINT (Ctrl-C) or SIGTERM

Changes

Core Changes

• runner.rs: Refactored run_suite to not return early on setup failure - instead it skips main tests but continues to teardown
• runner.rs: Added ignore_interruption parameter to run_corpus_file so teardown can run even when the interrupted flag is set
• main.rs: Added signal handler using ctrlc crate (with termination feature) to catch SIGINT, SIGTERM, and SIGHUP
• runner.rs: Added INTERRUPTED atomic flag and checks between tests to enable graceful shutdown

New Tests

• test/teardown_always/ - comprehensive tests for teardown-always-runs behavior
  • Unix tests using /tmp marker files
  • Windows tests using %TEMP% marker files
  • Verifies setup runs, teardown runs, and main tests are skipped when setup fails

Documentation

• Updated README.md to clarify teardown behavior

Limitations

• SIGKILL: Cannot be caught (kernel-level immediate termination)
• Signal handling in subshells: When cctr runs inside bash -c or as a background process, signal handling may not work reliably. Works correctly when cctr runs directly.

Testing

All existing tests pass plus 6 new Unix tests and 5 new Windows tests (platform-skipped appropriately).