net: add synchronous, role-neutral net.BoundHandle

[guybedford]

Add net.BoundHandle, a synchronous TCP bind primitive that mirrors POSIX bind(2): the socket is bound to a local address but stays role-agnostic until it is adopted as a server (server.listen()) or a client (new net.Socket({ handle }) followed by connect()).

Constructing a net.BoundHandle binds inline via the existing uv_tcp_bind() path, so the kernel-assigned address (including the ephemeral port when port is 0) is available immediately via boundHandle.address(), and bind errors throw synchronously.

Adoption transfers ownership of the underlying handle; afterwards address() and close() throw ERR_SOCKET_HANDLE_ADOPTED. An un-adopted handle is released with close() or via Symbol.dispose (using).

The handle is passed in-process rather than as a file descriptor, so it also works on Windows.

[nodejs-github-bot]

Review requested:

• @nodejs/net

[mcollina]

I don't understand why this is needed

[Ethan-Arrowood]

I appreciate this change. net.BoundHandle lets you reserve a TCP port and actually hold onto it until you're ready to use it. This simplifies utility modules like [get-port](https://npmx.dev/package/get-port) that are susceptible to TOCTOU (Time-of-Check to Time-of-Use) race conditions. Of course, it doesn't solve the problem perfectly since this is in-process only, but I think it's a great improvement.

As a follow-up, it'd be great if BoundHandle could optionally expose its underlying file descriptor, so the reservation can be handed to another process (Unix-only, since this relies on POSIX fd passing). That would extend the same race-free guarantee across a process boundary.

[guybedford]

Thanks for clarifying the use case here @Ethan-Arrowood. I've gone ahead and added a commit to expose the fd() as well here for the additional thread-passing use case.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/74217/