Real MQTT wiring for Bambu: connect, auto-detect v5/v3.1.1, request/reply

Removes GenericPrinter::receive (two disconnected fire-and-forget stubs
that didn't actually give request/response) in favor of a real mechanism
in the new src/printer/bambu_mqtt.rs:

- BambuMqttSession wraps whichever rumqttc client a connection ended up
  using (v3.1.1's AsyncClient/EventLoop for BambuMQTT v1, v5's for
  BambuMQTT v2 - different types, different event/error shapes) behind
  one interface. `open()` respects ForceV1/ForceV2, and for Auto tries
  v5 first, falling back to v3.1.1 on any failure (see the module doc
  comment for why "any failure", not a precisely-matched version-
  rejection code - unverified against a live printer, no broker here to
  test against).
- Reuses BambuTls::build_connector (now pub(crate)) for rumqttc's TLS
  transport via TlsConfiguration::NativeConnector, instead of a second,
  separate TLS trust story - hence rumqttc now pulls in use-native-tls
  instead of its default rustls stack, so there's one TLS implementation
  in the binary, not two.
- request() publishes and waits (with a timeout) for a report whose
  sequence_id matches, correlated via a shared pending-request table
  that a spawned background task completes as reports arrive - the
  first real tokio::spawn in this crate, flagged as deferred since
  printer/mod.rs's very first doc comment; this is why it was deferred
  until now, and why it's needed now (polling the eventloop has to run
  continuously, independent of whatever call site is waiting).
- publish()/publish_raw() are the fire-and-forget half, for commands
  like gcode_line that only get acknowledged implicitly.

BambuPrinter::connect/send_gcode_line/publish now call the real session
instead of printing what they would do. Verified against 192.0.2.1 (a
reserved, never-routable test address): both AsyncClient/EventLoop pairs
actually attempt a real TCP connection and fail gracefully (network
timeout, not a panic) when nothing answers, and Auto genuinely attempts
both while ForceV1 skips straight to v3.1.1.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MWQVtBGxHYLkVw2kBupd5g

src/printer/mod.rs

@@ -28,6 +28,7 @@
 
 mod bambu;
 mod bambu_commands;
+mod bambu_mqtt;
 mod klipper;
 mod marlin_serial;
 mod prusa;
@@ -73,7 +74,6 @@ pub trait GenericPrinter {
 
 
     async fn publish(&mut self, payload: String) -> Result<(), PrinterError>;
-    async fn receive(&mut self, payload: String) -> Result<(), PrinterError>;
 
     /// REQUIRED, no default body: `percent` is 0-100, but how it actually
     /// gets sent - an MQTT gcode-injection command, a REST call, raw bytes
@@ -159,15 +159,6 @@ impl GenericPrinter for PrinterHandle {
         }
     }
 
-    async fn receive(&mut self, payload: String) -> Result<(), PrinterError> {
-        match self {
-            PrinterHandle::Bambu(p) => p.receive(payload).await,
-            PrinterHandle::PrusaLink(p) => p.receive(payload).await,
-            PrinterHandle::PrusaSerial(p) => p.receive(payload).await,
-            PrinterHandle::Klipper(p) => p.receive(payload).await,
-        }
-    }
-
     async fn set_fan_speed(&mut self, percent: u8) -> Result<(), PrinterError> {
         match self {
             PrinterHandle::Bambu(p) => p.set_fan_speed(percent).await,