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).

tests/uplink_protocol.rs

@@ -106,7 +106,7 @@ fn a_vendor_specific_control_verb_is_rejected() {
 fn a_start_print_command_arrives_with_its_fields_intact() {
     let raw = r#"{"type":"command","command_id":"c","printer_id":"p1p","command":{"action":"start_print","job_id":"5b0e7c1a-0000-4000-8000-000000000000","file_name":"plate_1.3mf","url":"https://objects.example.com/jobs/abc.3mf?X-Amz-Signature=deadbeef&X-Amz-Expires=300"}}"#;
     match serde_json::from_str::<ServerMessage>(raw).expect("should parse") {
-        ServerMessage::Command { command: PrinterCommand::StartPrint { job_id, file_name, url }, .. } => {
+        ServerMessage::Command { command: PrinterCommand::StartPrint { job_id, file_name, url, .. }, .. } => {
             assert_eq!(job_id, "5b0e7c1a-0000-4000-8000-000000000000");
             assert_eq!(file_name, "plate_1.3mf");
             assert_eq!(