Split BambuPrinter into BambuGenericPrinter -> BambuV1Printer/BambuV2Printer

Completes the GenericPrinter -> BambuGenericPrinter -> BambuV1/V2 chain this
project wanted from the start. BambuGenericPrinter holds the fields and
behavior every Bambu printer shares (access code, serial number, CA trust,
the TLS test/fetch methods); BambuV1Printer and BambuV2Printer each *have*
one (composition) and are now genuinely separate types, ready to carry
flavour-specific report-schema fields later. Also threads through a new
'sn' (serial number) field the real MQTT topics will need.

Adds real 'fetch the CA automatically' capability, verified against a live
TLS server (not just compiled):
- BambuGenericPrinter::fetch_certificate() does trust-on-first-connect —
  connects once with verification disabled, captures the certificate the
  printer actually presents via native_tls's peer_certificate()/to_der(),
  and returns it as PEM. Deliberately a method you call once by hand
  (examples/fetch_bambu_cert.rs), not something connect() falls back to
  silently, since TOFU trusts whoever's on the network the moment you run
  it. Verified end-to-end against a local openssl s_server: the fetched
  PEM's SHA-256 fingerprint exactly matched the server's real certificate.
- Verified the other direction too: test_bambu_certs (bundled-CA mode)
  correctly REJECTS that same test server's cert, since it wasn't signed
  by the real Bambu CA.

Splitting BambuV1Printer/BambuV2Printer into distinct types broke the
PrinterHandle::BambuV1(x) | PrinterHandle::BambuV2(x) or-pattern in both
examples (or-patterns require every alternative to bind the same type) —
fixed by giving each variant its own match arm.

src/printer/mod.rs

@@ -2,6 +2,13 @@
 //! `GenericPrinter -> BambuV1/BambuV2/PrusaLink/PrusaSerial/Klipper`-style
 //! inheritance, since Rust structs can't `extend` each other.
 //!
+//! Bambu goes one level deeper than the others: `bambu.rs` has
+//! `BambuGenericPrinter` (fields + behavior every Bambu printer shares —
+//! access code, CA trust, the TLS handshake test/fetch) which
+//! `BambuV1Printer`/`BambuV2Printer` each *have* one of, mirroring the
+//! `GenericPrinter -> BambuGenericPrinter -> BambuV1/V2` chain this project
+//! actually wanted from the start — see that file's doc comment.
+//!
 //! Three pieces, each doing one job an OOP base class would normally do:
 //!
 //! 1. `PrinterBase` — a plain struct holding the shared *fields*. Every
@@ -25,7 +32,7 @@ mod bambu;
 mod klipper;
 mod prusa;
 
-pub use bambu::{BambuPrinter, BambuTls};
+pub use bambu::{BambuGenericPrinter, BambuTls, BambuV1Printer, BambuV2Printer};
 pub use klipper::KlipperPrinter;
 pub use prusa::{PrusaLinkPrinter, PrusaSerialPrinter};
 
@@ -79,8 +86,8 @@ pub enum PrinterFlavour {
 
 /// The closed set of printer kinds this fleet can talk to.
 pub enum PrinterHandle {
-    BambuV1(BambuPrinter),
-    BambuV2(BambuPrinter),
+    BambuV1(BambuV1Printer),
+    BambuV2(BambuV2Printer),
     PrusaLink(PrusaLinkPrinter),
     PrusaSerial(PrusaSerialPrinter),
     Klipper(KlipperPrinter),