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).

src/printer/klipper.rs

@@ -29,6 +29,15 @@ impl GenericPrinter for KlipperPrinter {
         Ok(())
     }
 
+    async fn publish(&mut self, payload: String) -> Result<(), PrinterError> {
+        // TODO: add publishing via HTTP
+        Ok(())
+    }
+
+    async fn receive(&mut self, payload: String) -> Result<(), PrinterError> {
+        // TODO: add HTTP response handling
+        Ok(())
+    }
     /// OVERRIDE: Moonraker's own **hypothetical** native API, not gcode injection - no
     /// percent-to-PWM conversion needed since it already takes a
     /// percentage directly. Real version:
@@ -37,4 +46,20 @@ impl GenericPrinter for KlipperPrinter {
         println!("[{}] Moonraker: SET_FAN_SPEED SPEED={:.2}", self.base.name, percent as f32 / 100.0);
         Ok(())
     }
+
+    /// OVERRIDE: real version is a multipart `POST /server/files/upload`
+    /// with `data` as the file part - Moonraker stores it on the printer's
+    /// virtual SD card, same upload-then-start model as Bambu/PrusaLink.
+    async fn upload_project_file(&mut self, file_name: String, data: Vec<u8>) -> Result<(), PrinterError> {
+        println!("[{}] POST /server/files/upload {file_name} ({} bytes)", self.base.name, data.len());
+        Ok(())
+    }
+
+    /// OVERRIDE: Moonraker's gcode-script endpoint, one line per call - same
+    /// idea as `set_fan_speed`'s `SET_FAN_SPEED` call above, just with an
+    /// arbitrary line instead of a hardcoded one.
+    async fn send_gcode_line(&mut self, gcode_line: String) -> Result<(), PrinterError> {
+        println!("[{}] POST /printer/gcode/script?script={gcode_line}", self.base.name);
+        Ok(())
+    }
 }