Capture selector wait errors immediately

[samuel-williams-shopify]

Summary

• capture errno in the EPoll and KQueue callback immediately after epoll_pwait2 / epoll_wait / kevent
• seed skipped callbacks with EINTR, so they cannot consume ambient thread-local errno
• raise from the captured error after returning through Ruby
• extend the pending-interrupt regression coverage by seeding an unrelated ENOENT before selection

Closes #205.

Tests

• Ruby 4.0.5/Linux EPoll: focused interruptable selector tests (6 passed, 10 assertions)
• Ruby 4.0.6/macOS KQueue: focused interruptable selector tests (6 passed, 10 assertions)
• Ruby 4.0.6/macOS: full suite (279 passed, 9 skipped, 901 assertions)