Prefix generated sequence_id instead of offsetting it

A printer's own free-running counter for unsolicited messages is always
a plain numeric string, so a non-numeric prefix ("cxproxy-") rules out
collision with it outright, rather than a large numeric starting offset
merely making collision improbable. Confirmed sequence_id need not be
numeric (live test, ISO-timestamp-prefixed value round-tripped fine) and
re-verified the new prefixed IDs round-trip through get_version on both
P1P (v1) and H2C (v2).

src/printer/bambu_mqtt.rs

@@ -86,6 +86,20 @@ const REQUEST_TIMEOUT: Duration = Duration::from_secs(10);
 /// 1MB has real headroom for that.
 const MAX_PACKET_SIZE: usize = 1024 * 1024;
 
+/// Every `sequence_id` this crate generates gets this prefix - a printer
+/// runs its own independent counter for unsolicited messages (confirmed
+/// live: `print.push_status`'s was already in the 500s at the start of a
+/// session, climbing steadily), and that counter has only ever been
+/// observed as a plain unprefixed integer string. An earlier version of
+/// this just started our own counter from a large offset instead of
+/// prefixing it, to dodge the same range a printer's counter climbs
+/// through - confirmed live (with an ISO-timestamp-prefixed sequence_id)
+/// that `sequence_id` isn't required to be numeric at all, so a
+/// non-numeric prefix rules out collision outright instead of merely
+/// making it improbable, and the counter itself can go back to starting
+/// at 1 each session.
+const SEQUENCE_ID_PREFIX: &str = "cxproxy-";
+
 enum BambuMqttClient {
     V1(rumqttc::AsyncClient),
     V2(rumqttc::v5::AsyncClient),
@@ -292,7 +306,10 @@ impl BambuMqttSession {
     /// `PrintStatusCache`. Not for a command with no reply at all either,
     /// like `gcode_line` - use `publish` (fire-and-forget) for those.
     pub async fn request(&self, category: &str, mut body: Value) -> Result<Value, PrinterError> {
-        let sequence_id = self.next_sequence_id.fetch_add(1, Ordering::Relaxed).to_string();
+        let sequence_id = format!(
+            "{SEQUENCE_ID_PREFIX}{}",
+            self.next_sequence_id.fetch_add(1, Ordering::Relaxed)
+        );
         body["sequence_id"] = Value::String(sequence_id.clone());
 
         let (tx, rx) = oneshot::channel();