[controller][feat] add realtime snapshot

[mr-chelyshkin]

📝 Description

Adds controller_snapshot() to SupervisorHandle;
Add event duration.

✅ Checklist

• Documentation updated (README, rustdoc, examples)
• Tests added/updated (if relevant)
• No breaking changes introduced
• CI passes locally

Examples

if let Some(snap) = handle.controller_snapshot().await {
    println!(
        "running={}, queued={}",
        snap.running_count(),
        snap.total_queued()
    );

    if let Some(slot) = snap.slot("deploy") {
        println!(
            "deploy: {:?}, depth={}",
            slot.status,
            slot.queue_depth
        );
    }
}