Fix io_uring scheduler v4 compilation

[samuel-williams-shopify]

Summary

• Keep the legacy io_read, io_pread, io_write, and io_pwrite implementations entirely within their scheduler-version conditional branches.
• Make native extension build failures propagate from the Bake build task.

Background

The scheduler interface v4 branches returned through the locked-buffer callbacks, but four premature #endif directives also compiled the legacy implementations without their local variables. This broke io-event 1.20.0 on Linux with liburing under Ruby 4.0 and Ruby head.

The existing io-event CI did encounter these compiler errors, but bake.rb#build ignored the failed system results. Tests then continued with the pure-Ruby selector, allowing the jobs to report success. Raising on failed build commands ensures future native compilation failures fail CI.

This was observed in socketry/async#469.

Testing

• Ruby 4.0.5 on macOS: 289 passed, 15 skipped (910 assertions).
• Linux Ruby 4.0/head with liburing will be exercised by CI.