Compare producer stop markers by identity

[OskarEichler]

Summary

Recognize only the Parallel::Stop object as the producer terminator.

Reproduction and verification

A producer yielding an ordinary object whose == method returns true currently stops without processing it. Stop is a unique sentinel, so its identity must be compared rather than delegating equality to the item.

A finite producer fixture with custom equality fails before and passes after. Fifteen existing producer/queue examples pass.

The combined installed-release candidate passes 800 model checks across direct, thread, process and Ractor modes. Existing selected suite: 165 examples, zero failures, nine pending, baseline and candidate on rbenv Ruby 4.0.6. Global process-name kill tests and the 200-process stress example were excluded for shared-host safety. SQLite examples use temporary databases. An external verification bundle uses already-installed dependencies; upstream Gemfile/lockfile/tests remain unchanged.

Runtime RuboCop passes. Full upstream RuboCop reports the same pre-existing RSpec/MatchWithSimpleRegex offense in spec/parallel_spec.rb:278 on both versions; no full green-lint claim. Gem packaging retains 2.1.0, Ruby >= 3.3, all four published paths and metadata, with no dependency changes.

Compatibility and limitations

Items merely no longer impersonate Stop via custom equality. Returning Parallel::Stop still terminates production; no producer scheduling change.

No tests were added or modified under the consuming repository's no-new-tests policy. Reproductions/model checks are external scratch scripts. No production operations or claim of complete concurrency/platform coverage.

[grosser]

a bit far-fetched but technically correct 👍