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.

Cargo.toml

@@ -21,6 +21,10 @@ path = "examples/printer_polymorphism.rs"
 name = "test_bambu_certs"
 path = "examples/test_bambu_certs.rs"
 
+[[example]]
+name = "fetch_bambu_cert"
+path = "examples/fetch_bambu_cert.rs"
+
 [dependencies]
 tokio = { version = "1.40", features = ["full"] }
 tokio-tungstenite = { version = "0.24", features = ["native-tls"] }
@@ -29,6 +33,7 @@ serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 toml = "0.8"
 native-tls = "0.2"
+base64 = "0.22"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter", "json"] }
 anyhow = "1"