Add Windows IOCP selector

[samuel-williams-shopify]

Summary

• Add an experimental Windows selector backed by IOCP for overlapped operations and wakeups.
• Support practical socket-oriented io_wait, including a Winsock readiness fallback for writable sockets where IOCP does not provide readiness semantics directly.
• Wire the Windows selector into the extension build/debug selector wrapper, skip documented Windows readiness edge cases, and disable WorkerPool on Windows because it depends on POSIX pthread semantics.

Windows readiness contract

This is intended as a pragmatic Windows baseline, not perfect POSIX parity. The main supported target is socket scheduling.

• Disk files and character handles are treated as immediately ready; the actual read/write operation reports real errors.
• Non-socket writable waits are optimistic, because Windows does not expose a general readiness primitive for arbitrary handles.
• Pipe-readable waits may use overlapped ReadFile when the handle supports it.
• Mixed READABLE | WRITABLE waits return any immediately-ready writable/error state first; otherwise the selector arms the readable wait.

Tests

• bundle exec bake test
• bundle exec rubocop
• git diff --check
• GitHub Actions: Windows MRI 3.3, 3.4, and 4.0 passed on 1a12594.
• JRuby and TruffleRuby failures are expected current noise.