Bound MQTT publishes and send status refreshes unacked

On the first real dispatch the proxy froze about 4.5 minutes in: pushall was
published at QoS 1 from two tasks while holding the fleet lock, and once the
in-flight window filled every publish blocked forever, stalling telemetry,
heartbeats and the uplink. Publishes now time out after 10 s and pushall is
QoS 0.

Also adds an optional ams_slot to start_print so a job can choose its AMS
slot (single-filament projects, same material type only).

examples/bambu_mapping_check.rs

@@ -47,6 +47,10 @@ async fn main() -> anyhow::Result<()> {
     for f in continuum_proxy::printer::plate_filaments(&bytes, 1)? {
         println!("project filament {}: {} {} #{}", f.index, f.filament_type, f.tray_info_idx, f.color);
     }
+    if let Some(slot) = std::env::var("AMS_SLOT").ok().and_then(|v| v.parse::<u8>().ok()) {
+        println!("(with AMS_SLOT={slot} chosen explicitly)");
+        bambu.set_ams_slot_override(Some(slot));
+    }
     match bambu.filament_source_for(&bytes, 1).await {
         Ok(source) => println!("mapping the production path would use: {source:?}"),
         Err(err) => println!("the production path would REFUSE: {err}"),