Provisioners: replace blind sleeps in proxmox_lxc create/TUN path with readiness checks

[Zorlin]

What

Two blind sleeps remain in proxmox_lxc.rs's create/TUN path — pre-existing antipatterns left out of #28 (which only fixed the destroy footgun). They violate the repo's "no sleeps" rule and make create timing fragile/host-dependent.

In ensure_exists, the needs_tun block (src/provisioners/proxmox_lxc.rs:1465):

if needs_tun {
    // Wait for container to be fully created
    std::thread::sleep(std::time::Duration::from_secs(2));   // line 1467 — blind
    let _ = self.configure_tun(&conn, vmid)?;
    // Now start the container
    self.start_container(&conn, vmid)?;
    std::thread::sleep(std::time::Duration::from_secs(3));   // line 1471 — blind
}

• 1467 (2s): "wait for container to be fully created" — a guess at how long the async create takes.
• 1471 (3s): after start_container — a guess at how long start takes.

Both are unconditional; on a slow node they race (configure_tun / start hits a not-yet-ready container), on a fast node they waste time.

Fix

Replace each with a readiness check (no blind sleeps):

• The create and start endpoints return a Proxmox UPID (async task). The cleanest fix is to poll the task to completion — there's already a task-wait poll pattern in this file (~line 918, polls /tasks/{upid}/status until status == "stopped"). Generalise/reuse it.
• Alternatively, poll the container's /status/current until it reports the desired state (created/running) — the same shape as the wait_for_stopped readiness poll added in feat(provisioners): safe destroy state model (stopped/absent/destroyed) #29 (proxmox_vm.rs / proxmox_lxc.rs).

Scope note

These are unrelated to the destroy footgun (#28 / #29), which is why they were deferred. The other sleeps in this file are fine: ~918 and ~866 are conditional polls (task/template readiness with attempt caps), and ~1282 is #29's bounded wait_for_stopped.

Acceptance

• grep -rn "thread::sleep\|time::sleep" src/provisioners/ shows only bounded, conditional readiness polls (no unconditional from_secs(N)).
• cargo test / clippy / fmt / build green.
• A TUN-provisioning create still configures TUN + starts reliably (ideally exercised against a real cluster).