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/bambu_commands/mod.rs

@@ -49,6 +49,12 @@ pub struct CommandEnvelope {
 /// category can have more than one possible request shape - if that ever
 /// happens here, that variant's payload becomes its own small
 /// `#[serde(tag = "command")]` enum instead of a single struct.
+///
+/// The "print" category isn't a variant here at all: unlike `info`/
+/// `pushing`, it has many possible request shapes (`gcode_line`, `pause`,
+/// `resume`, ...) in continuum-schemas' own `oneOf`, so
+/// `bambu.rs`'s `send_gcode_line_impl` builds that one directly via
+/// `continuum-schema-instance` instead of adding it here.
 #[derive(serde::Serialize, Debug)]
 #[serde(rename_all = "snake_case")]
 pub enum BambuRequest {