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).

printers.example.toml

@@ -5,7 +5,9 @@
 [[printer]]
 id = "p1p"
 name = "P1P"
-vendor = "bambu_v1"                    # bambu_v1 | bambu_v2 | prusa_link | prusa_serial | klipper
+# Forced instead of "bambu" here since this one's a known-old printer -
+# see fleet.rs's PrinterEntry doc comment for the full vendor list.
+vendor = "bambu_v1"
 host = "192.168.1.50"
 access_code = "REPLACE_WITH_ACCESS_CODE"
 sn = "REPLACE_WITH_SERIAL_NUMBER"      # printer screen > Settings > Device (used for MQTT topics later)
@@ -18,7 +20,10 @@ ca_cert_path = "./certs/p1p.pem"
 [[printer]]
 id = "h2c"
 name = "H2C"
-vendor = "bambu_v2"
+# "bambu" auto-detects MQTT v5-vs-v3.1.1 (BambuMQTT v2-vs-v1) on connect -
+# the recommended default, since a firmware update can change which one a
+# given printer speaks without you knowing ahead of time.
+vendor = "bambu"
 host = "192.168.1.51"
 access_code = "REPLACE_WITH_ACCESS_CODE"
 sn = "REPLACE_WITH_SERIAL_NUMBER"
@@ -37,3 +42,10 @@ sn = "REPLACE_WITH_SERIAL_NUMBER"
 # vendor = "prusa_link"
 # host = "192.168.1.53"
 # api_key = "REPLACE_WITH_API_KEY"
+
+# [[printer]]
+# id = "mk3s"
+# name = "MK3S+"
+# vendor = "prusa_serial"
+# port = "/dev/ttyACM0"
+# baud_rate = 115200            # optional, defaults to 115200