Add upload_project_file/send_gcode_line, real Marlin serial, Bambu protocol auto-detect

- GenericPrinter grows upload_project_file, send_gcode_line, and a
  generic upload_gcode<P> driver that streams a whole gcode file line by
  line through any vendor's send_gcode_line.
- PrusaSerialPrinter's connect/send_gcode_line are real (not stubs): a
  held-open MarlinSerialConnection (src/printer/marlin_serial.rs) writes
  each line and waits for Marlin's "ok" before sending the next. Config
  changed from com_port: i16 to port: String + baud_rate: u32 to hold a
  real device path.
- Bambu: BambuGenericPrinter + BambuV1Printer/BambuV2Printer collapsed
  into one BambuPrinter, since which MQTT/BambuMQTT version (v3.1.1/v1
  vs v5/v2) a printer speaks is discovered at connect() time (or forced),
  not something config should pick a type for - the same physical
  printer can change after a firmware update. printers.toml gets a new
  "bambu" vendor (auto-detect, the default) alongside bambu_v1/bambu_v2
  (force a specific one).
- Bambu's gcode_line request is now built for real via
  continuum-schema-instance against continuum-schemas' JSON Schema,
  picking the v1 or v2 schema directory based on the detected protocol
  version, instead of a hand-written serde struct.
- Added rumqttc as a dependency (not wired up yet - the real MQTT
  v5-then-v3.1.1 fallback connect and the publish/receive plumbing are
  still stubs).

examples/fetch_bambu_cert.rs

@@ -23,8 +23,7 @@ fn main() -> anyhow::Result<()> {
     let pem = printers
         .iter()
         .find_map(|printer| match printer {
-            PrinterHandle::BambuV1(bambu) if bambu.base().id == printer_id => Some(bambu.fetch_certificate()),
-            PrinterHandle::BambuV2(bambu) if bambu.base().id == printer_id => Some(bambu.fetch_certificate()),
+            PrinterHandle::Bambu(bambu) if bambu.base().id == printer_id => Some(bambu.fetch_certificate()),
             _ => None,
         })
         .ok_or_else(|| anyhow::anyhow!("no Bambu printer with id '{printer_id}' in printers.toml"))??;