Fix io_uring IO hooks for scheduler interface v4

[samuel-williams-shopify]

Ruby scheduler interface v4 changed the IO hook argument order and made each hook a single-operation primitive. The io_uring backend was still being tested and driven as though it should return EAGAIN from the event-loop fiber, which caused the ruby-head failures introduced by ruby/ruby#18483.

This change:

• attempts one non-blocking syscall first and returns immediately when it completes;
• submits one read or write SQE and suspends the calling child fiber only after EAGAIN/EWOULDBLOCK;
• defers SQE submission so operations can be batched by the event loop;
• uses the new locked IO::Buffer callbacks so kernel operations cannot outlive their buffers;
• waits for an interrupted operation to complete or cancel before releasing its buffer lock;
• keeps completion records alive until both operation and cancellation CQEs are observed; and
• updates scheduler-interface-v4 tests to run genuinely suspending hooks from child fibers while preserving legacy-interface behavior.

Validation:

• dedicated URing benchmark passes on Ubuntu with ruby-head
• ruby-head test job passes on Ubuntu
• 289 local tests pass, with 15 platform skips and 910 assertions
• RuboCop and git diff --check pass

The coverage failure is also present on main; the JRuby failure is an unrelated unsupported-fork failure.