feat: reuse the dogstatd socket fd

[sky-mart]

The motivation is to be able to write like this in orb-core: (the rest is collateral damage)

Parent process (orb-core main):

use std::os::fd::AsRawFd;
use std::time::Duration;
use orb_dogd::DogstatsdClient;

let client = DogstatsdClient::new();           // returns immediately;
                                               // bg thread starts retrying

// ... emit metrics any time; they buffer in the bounded channel
//     until the bg thread connects, then drain out
client.incr("orb.boot", ["stage:early"])?;

// Before forking a subprocess agent, get a shareable FD.
// Blocks until the bg thread has actually connected (or 5min timeout).
let fd = client
    .wait_for_connection(Duration::from_secs(300))
    .expect("dsd unreachable for 5 minutes");

// Hand `fd` to the child: clear CLOEXEC and stash its number in an env var.
// (This part lives in orb-core's ProcessInitializer; orb-dogd doesn't
// dictate how you pass the FD — it just gives you one.)
clear_descriptor_cloexec(&fd)?;
env::set_var("ORB_CORE_PROCESS_DOGSTATSD", fd.as_raw_fd().to_string());
// ... then fork+exec / agentwire spawn

Subprocess (forked agent, inside the sandbox netns):

use std::os::fd::FromRawFd;
use std::os::unix::net::UnixDatagram;
use orb_dogd::DogstatsdClient;

let raw_fd: i32 = env::var("ORB_CORE_PROCESS_DOGSTATSD")?.parse()?;
// SAFETY: the FD was inherited across exec; we own it.
let socket = unsafe { UnixDatagram::from_raw_fd(raw_fd) };

let client = DogstatsdClient::from_unix_datagram(socket);
// No retry, no connect — `socket` is already connected to the
// parent's dsd session.

client.gauge("orb.agent.ready", 1.0, ["agent:iris"])?;