feat: Support for running simply sequentially without worker threads

[dazuma]

No description provided.

[Copilot]

Pull request overview

This PR adds support for running the Ractor::Wrapper sequentially without worker threads, changing the default behavior from single-threaded to sequential execution mode.

• Changes the default threads parameter from 1 to 0, disabling worker threads by default
• Implements sequential execution logic that handles method calls directly in the main loop when threads are disabled
• Expands test coverage to verify all functionality works in both sequential (threads: 0) and threaded (threads: 2) modes

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
lib/ractor/wrapper.rb	Implements sequential execution mode, updates default threads parameter to 0, adds port closing for resource cleanup, changes send to __send__ for more robust method invocation, and updates documentation to reflect new threading model
test/test_wrapper.rb	Restructures tests to run with both sequential and threaded configurations, ensuring comprehensive coverage of all wrapper behaviors in both modes
README.md	Updates documentation to clarify threading model caveats and align with code changes

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