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).

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

examples/test_bambu_certs.rs

@@ -12,13 +12,9 @@ use continuum_proxy::printer::{GenericPrinter, PrinterHandle};
 fn main() -> anyhow::Result<()> {
     let printers = fleet::load(std::path::Path::new("printers.toml"))?;
 
-    // BambuV1Printer and BambuV2Printer are separate types (each *has* a
-    // BambuGenericPrinter rather than being the same struct), so this
-    // needs one arm per variant rather than a single `A(x) | B(x)` pattern.
     for printer in &printers {
         match printer {
-            PrinterHandle::BambuV1(bambu) => report(bambu.base().name.as_str(), bambu.test_tls_handshake()),
-            PrinterHandle::BambuV2(bambu) => report(bambu.base().name.as_str(), bambu.test_tls_handshake()),
+            PrinterHandle::Bambu(bambu) => report(bambu.base().name.as_str(), bambu.test_tls_handshake()),
             _ => {}
         }
     }