Refactor async poll()/select() onto a per-inode readiness wait-queue

[guybedford]

This refactors out a partial base from #27207.

Move the readiness wait-queue onto FSNode (addListener/notifyListeners) so dup'd fds share one queue, and rewrite the suspending poll() path (readPollfds/writePollfds/pollWait) on top of it. Producers (SOCKFS, PIPEFS) now notify the node on readiness transitions, and close() wakes waiters with POLLNVAL.

• FSNode gains addListener(cb, exclusive) / notifyListeners(flags) — a per-inode Set of listener entries. It lives on the node (not the fd) so dup'd fds share one queue. Only nodes with real readiness (sockets, pipes) ever populate it; always-ready types (regular files, ttys) never touch it.
• The suspending poll() path is rewritten on top of it (readPollfds/writePollfds/pollWait/pollOne), replacing the old doPollAsync + makeNotifyCallback machinery.
• Producers now notify the node on a readiness transition: PIPEFS on writes, SOCKFS via its emit bridge, and close() wakes any waiter with POLLNVAL.
• Socket hangup semantics tightened: a peer half-close is POLLRDHUP (only a fully closed connection is POLLHUP), and a queued client makes a listening socket POLLIN.

The stream_ops.poll backend handler signature changes from poll(stream, timeout) to poll(stream) returning the current readiness mask.

Note: the exclusive parameter on addListener and the round-robin wakeup path in notifyListeners are deliberately layered in here as groundwork for the EPOLLEXCLUSIVE handling that lands in the epoll followup (#27207). No caller passes exclusive in this PR yet, so that branch is currently inert — it's included now to keep the wait-queue API stable across the split.

[sbc100]

Great! Thanks for splitting this out.

lgtm with some comments

[sbc100]

Looks great!