Add io_pread and io_pwrite methods to IO_Event_Selector_KQueue

[ItaloCobains]

Types of Changes

• New feature.

Contribution

• I added tests for my changes.
• I tested my changes locally.
• I agree to the Developer's Certificate of Origin 1.1.

[Copilot]

Pull request overview

This PR adds positional IO buffer operations (io_pread/io_pwrite) to the IO::Event::Selector::KQueue backend, aligning KQueue with other selectors that support IO::Buffer-based scheduler hooks.

Changes:

• Implement IO_Event_Selector_KQueue_io_pread using pread(2) with scheduler-aware retry via io_wait.
• Implement IO_Event_Selector_KQueue_io_pwrite using pwrite(2) with scheduler-aware retry via io_wait.
• Expose both methods to Ruby via rb_define_method when HAVE_RUBY_IO_BUFFER_H is available.