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).

src/uplink/mod.rs

@@ -652,7 +652,7 @@ async fn dispatch(fleet: &SharedFleet, printer_id: &str, command: PrinterCommand
         PrinterCommand::ControlPrint { control } => {
             printer.control_print(control).await.map_err(|err| err.to_string())
         }
-        PrinterCommand::StartPrint { job_id, file_name, .. } => {
+        PrinterCommand::StartPrint { job_id, file_name, ams_slot, .. } => {
             let bytes = downloaded.ok_or_else(|| "internal error: start_print reached dispatch with no downloaded file".to_string())?;
             let byte_count = bytes.len();
             info!(%printer_id, %job_id, %file_name, byte_count, "job file downloaded, sending to printer");
@@ -661,10 +661,19 @@ async fn dispatch(fleet: &SharedFleet, printer_id: &str, command: PrinterCommand
             // all as one error - so the stage here is "printer", not finer.
             // The printer's own message says which part (mapping, upload of
             // the file, or start) it was.
-            printer
+            if let PrinterHandle::Bambu(bambu) = &mut *printer {
+                bambu.set_ams_slot_override(ams_slot);
+            } else if ams_slot.is_some() {
+                return Err("this printer cannot be told which AMS slot to use".into());
+            }
+            let result = printer
                 .upload_project_file(file_name, bytes)
                 .await
-                .map_err(|err| format!("printer upload/start failed (download of {byte_count} bytes succeeded): {err}"))
+                .map_err(|err| format!("printer upload/start failed (download of {byte_count} bytes succeeded): {err}"));
+            if let PrinterHandle::Bambu(bambu) = &mut *printer {
+                bambu.set_ams_slot_override(None);
+            }
+            result
         }
     }
 }