[fix] Socket: Fix stale indexes and surface close/error reliably in _hx_std_socket_poll_events

[dimensionscape]

This PR contains two related fixes in _hx_std_socket_poll_events to make polling safer and more consistent:

1. Clear ridx/widx on entry and early return
   Prevents stale values when polling with empty registries or when select()/poll() errors out. Previously this could result in phantom indexes or poll([]) returning [null].

2. Surface close/error conditions reliably

   • Windows: now pass and handle an exceptfds set to detect errors and connection resets. On SOCKET_ERROR, all current read fds are surfaced so userland can clean up.
   • POSIX: include POLLERR | POLLNVAL alongside POLLIN/HUP and POLLOUT/HUP so teardown/error conditions are not missed.

Together these changes ensure that:

• Empty registries return no ready sockets.
• Closed/reset sockets always show up as "ready" for one tick so userland can detect EOF and deregister them.
• The behavior is consistent across Windows and POSIX backends.