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/printer_polymorphism.rs

@@ -4,36 +4,40 @@
 //! - `connect()` is called the same way regardless of vendor (polymorphism),
 //!   but each variant's `impl GenericPrinter` runs completely different
 //!   code (override).
-//! - `dispatch_gcode()` only exists on the two Bambu structs (extension) -
-//!   you have to `match` the enum back down to the concrete type to reach
-//!   it, which is the trade-off for not having implicit downcasting.
+//! - `dispatch_gcode()` only exists on the Bambu struct (extension) - you
+//!   have to `match` the enum back down to the concrete type to reach it,
+//!   which is the trade-off for not having implicit downcasting.
 
 use continuum_proxy::printer::{
-    BambuTls, BambuV1Printer, BambuV2Printer, GenericPrinter, KlipperPrinter, PrinterHandle, PrusaLinkPrinter,
-    PrusaSerialPrinter,
+    BambuPrinter, BambuProtocolPreference, BambuTls, GenericPrinter, KlipperPrinter, PrinterHandle,
+    PrusaLinkPrinter, PrusaSerialPrinter,
 };
 
 #[tokio::main]
 async fn main() {
     let mut fleet: Vec<PrinterHandle> = vec![
-        PrinterHandle::BambuV1(BambuV1Printer::new(
+        // Forced: a known-old printer.
+        PrinterHandle::Bambu(BambuPrinter::new(
             "p1",
             "P1S",
             "192.168.1.49",
             "87654321",
             "01P00A000000000",
             BambuTls::BundledCa,
+            BambuProtocolPreference::ForceV1,
         )),
-        PrinterHandle::BambuV2(BambuV2Printer::new(
+        // Auto: the recommended default - see bambu.rs's doc comment.
+        PrinterHandle::Bambu(BambuPrinter::new(
             "p2",
             "X1 Carbon",
             "192.168.1.50",
             "12345678",
             "00M00A000000000",
             BambuTls::BundledCa,
+            BambuProtocolPreference::Auto,
         )),
         PrinterHandle::PrusaLink(PrusaLinkPrinter::new("p3", "MK4", "192.168.1.51", "prusa-api-key")),
-        PrinterHandle::PrusaSerial(PrusaSerialPrinter::new("p4", "MK3S+", 0)),
+        PrinterHandle::PrusaSerial(PrusaSerialPrinter::new("p4", "MK3S+", "/dev/ttyACM0", 115_200)),
         PrinterHandle::Klipper(KlipperPrinter::new("p5", "Voron 2.4", "192.168.1.52")),
         // No host configured - this one will hit the Err path.
         PrinterHandle::Klipper(KlipperPrinter::new("p6", "Broken Voron", "")),
@@ -50,15 +54,11 @@ async fn main() {
     }
 
     // Vendor-only extension: reachable only after matching the concrete
-    // variant back out of the enum. BambuV1Printer and BambuV2Printer are
-    // separate types now (each *has* a BambuGenericPrinter rather than
-    // being the same struct), so - unlike before the split - this needs
-    // two arms instead of one `A(x) | B(x)` pattern: an or-pattern requires
-    // every alternative to bind the same type, and these no longer do.
+    // variant back out of the enum - one arm now that Bambu is a single
+    // type regardless of which protocol version `connect()` settled on.
     for printer in &fleet {
         match printer {
-            PrinterHandle::BambuV1(bambu) => bambu.dispatch_gcode("part.gcode.3mf"),
-            PrinterHandle::BambuV2(bambu) => bambu.dispatch_gcode("part.gcode.3mf"),
+            PrinterHandle::Bambu(bambu) => bambu.dispatch_gcode("part.gcode.3mf"),
             _ => {}
         }
     }