Add after_forks_complete configuration hook

[supairish]

Summary

Adds an after_forks_complete hook to Flatware::Configuration, called once in the sink process after all workers have finished and all work is complete (just before DRb.stop_service).

This complements the existing before_fork and after_fork hooks by providing a place to run teardown logic that should happen exactly once after the entire parallel run — for example, cleaning up shared test databases or aggregating results.

Usage

Flatware.configure do |config|
  config.after_forks_complete do
    # runs once after all workers finish
  end
end

Changes

• lib/flatware/configuration.rb — adds after_forks_complete accessor with the same pattern as before_fork / after_fork
• lib/flatware/sink.rb — calls the hook in check_finished! before stopping DRb
• spec/flatware/configuration_spec.rb — tests default noop behavior and block storage

Test plan

• Existing specs pass
• New spec verifies default is a callable noop
• New spec verifies the configured block is invoked

[supairish]

Note: the CI failure is a pre-existing RuboCop offense on master (lib/flatware/version.rb:4 — double quotes on the version string). It's unrelated to the changes in this PR.