Add real FTPS upload to Bambu printers

`upload_project_file` was a `println!`. MQTT can start a print but can
only name a file the printer already has, so moving the bytes there is a
separate protocol: implicit FTPS on port 990, user `bblp`, password the
same access code MQTT uses, into the printer's own `cache/` directory.

Confirmed live against a P1P: 102 bytes uploaded, byte count matched
exactly, first attempt.

Deliberately suppaftp's *blocking* client on `tokio::task::spawn_blocking`
rather than its async one. The blocking client takes a plain
`native_tls::TlsConnector`, so this reuses `BambuTls::build_connector()`
unchanged and every TLS trust decision for a printer stays in that one
method; the async client needs a second TLS stack (`async-native-tls`)
whose futures-vs-tokio `AsyncRead` mismatch has already cost this project
a debugging session. `connect()`'s doc comment already pointed at
`spawn_blocking` as the eventual answer for blocking work, and a
multi-megabyte upload earns it more obviously than a handshake does.

`connect_secure_implicit` sits behind suppaftp's `deprecated` feature
because implicit FTPS is what the RFCs deprecated in favour of explicit
AUTH TLS. The printers only speak implicit, so the feature is on.

Split into `upload_file` (FTPS only) and `start_uploaded_file` (the MQTT
command that begins printing), because the two halves have very different
consequences: moving a file onto a printer is inert and reversible, while
starting a print commits the machine to hours of physical work. That split
is what made the transfer testable against real hardware at all.

`start_uploaded_file` dispatches on file type - a `.3mf` is a container so
`print.project_file` needs both a `url` and a `param` naming the gcode
inside it, while a bare `.gcode` has nothing to select and
`print.gcode_file` takes just a path. It has deliberately never been
fired; its doc comment lists the three guesses to confirm first.

suppaftp is MIT OR Apache-2.0, checked against docs/LICENSES.md before
being added, along with all 22 crates it pulled in.

src/printer/mod.rs

@@ -22,12 +22,14 @@
 //! Run `cargo run --example printer_polymorphism` to see it in action.
 //!
 //! Real MQTT is built now - `bambu.rs`/`bambu_mqtt.rs` for Bambu's
-//! protocol, `marlin_serial.rs` for a real Marlin serial connection - see
-//! each module's own doc comment. PrusaLink/Klipper's HTTP clients are
-//! still stubs; that's the next piece to build past this module.
+//! protocol, `bambu_ftps.rs` for getting files onto a Bambu printer,
+//! `marlin_serial.rs` for a real Marlin serial connection - see each
+//! module's own doc comment. PrusaLink/Klipper's HTTP clients are still
+//! stubs; that's the next piece to build past this module.
 
 mod bambu;
 mod bambu_commands;
+mod bambu_ftps;
 mod bambu_mqtt;
 mod klipper;
 mod marlin_serial;