Add a test suite (RSpec or Minitest) with CI on Linux + Redis

[jerry7991]

Problem

The repository has no test framework wired up and no CI. This is the root cause that lets bugs like the capital-D require (Linux load failure) and the pool-size-after-build ordering slip through.

Fix

1. Pick a framework (RSpec is idiomatic for gems; Minitest is lighter).
2. Add a spec/ or test/ directory with coverage for:
   • Configuration defaults + ENV overrides.
   • YamlLoader applies values, tolerates missing file.
   • HandlerRegistry register / lookup / clear / uniqueness error.
   • Dispatcher.enqueue builds correct payload, rejects non-workers, pushes to the right key.
   • Worker DSL: queue_as, job_name, fallback to default queue.
   • End-to-end: enqueue → worker loop pops → handler runs (integration test against real Redis).
3. Add GitHub Actions CI on ubuntu-latest with a Redis service container. Run the full suite plus a smoke test that requires the gem (catches case-sensitive filename bugs).
4. Add RuboCop with the project's existing style directives.

Acceptance

• bundle exec rspec passes locally and in CI.
• CI runs on every PR against master.
• Linux load regression would fail CI.