Add post-suite hooks for SimpleCov integration

[cb341]

🤖 This PR was authored by Claude Haiku and Opus (Anthropic).

Problem

SimpleCov doesn't support rspec-conductor. When tests run across forked workers, each process writes its own coverage data, but there's no mechanism to merge results in the parent process after workers exit.

SimpleCov handles this for parallel_tests through explicit integration:

• CommandGuesser.from_env detects PARALLEL_TEST_GROUPS / TEST_ENV_NUMBER to assign unique command names per worker
• final_result_process? gates reporting to run only in the last ParallelTests process
• wait_for_other_processes calls ParallelTests.wait_for_other_processes_to_finish before merging

None of this works for rspec-conductor. Users are forced to manually invoke ResultMerger.merged_result and process_result after rspec-conductor exits, e.g. in CI:

# Merge and validate SimpleCov results from all workers
bundle exec ruby -r simplecov -e "
  result = SimpleCov::ResultMerger.merged_result
  if result
    result.format!
    SimpleCov.process_result(result)
  end
"

Related SimpleCov issues:

• #1156: rspec-conductor support (the direct request)
• #232: minimum_coverage validates too early, before all processes finish
• #350: merging parallel test results requires manual command_name / merge_timeout setup
• #1091: branch coverage data lost when merging across parallel runners (Flatware)

Solution

Add a post-suite hook mechanism: callbacks that run in the parent process after all workers complete, before printing the summary.

RSpec::Conductor.register_post_suite_hook do
  # runs in parent, after workers exit
end

When SimpleCov is defined, rspec-conductor automatically registers a hook that calls merged_result and process_result. No application-side code needed. This mirrors what SimpleCov does internally for parallel_tests via final_result_process? + wait_for_other_processes, but driven from the runner side.

Changes

• Server.post_suite_hooks / Server.register_post_suite_hook: class-level hook registry
• Server#run: call hooks after suite_run.suite_complete, before print_summary
• RSpec::Conductor.register_post_suite_hook: public API (delegates to Server)
• Auto-register SimpleCov hook when SimpleCov constant is defined (parent process only)