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/fleet.rs

@@ -7,7 +7,7 @@ use std::path::Path;
 
 use serde::Deserialize;
 
-use crate::printer::{BambuPrinter, BambuTls, KlipperPrinter, PrinterHandle, PrusaLinkPrinter, PrusaSerialPrinter};
+use crate::printer::{BambuTls, BambuV1Printer, BambuV2Printer, KlipperPrinter, PrinterHandle, PrusaLinkPrinter, PrusaSerialPrinter};
 
 #[derive(Debug, Deserialize)]
 struct FleetFile {
@@ -23,6 +23,9 @@ struct PrinterEntry {
     vendor: String,
     host: Option<String>,
     access_code: Option<String>,
+    /// The printer's serial number — Bambu's real MQTT topics are addressed
+    /// by serial (`device/{sn}/report`), not by our own `id`.
+    sn: Option<String>,
     api_key: Option<String>,
     com_port: Option<i16>,
     /// Path to a printer-specific CA certificate. Leave unset to use the
@@ -51,16 +54,19 @@ pub fn load(path: &Path) -> anyhow::Result<Vec<PrinterHandle>> {
 
 fn build_printer(entry: PrinterEntry) -> anyhow::Result<PrinterHandle> {
     match entry.vendor.as_str() {
-        "bambu_v1" | "bambu_v2" => {
+        "bambu_v1" => {
             let host = require_field(&entry.id, "host", entry.host)?;
             let access_code = require_field(&entry.id, "access_code", entry.access_code)?;
+            let sn = require_field(&entry.id, "sn", entry.sn)?;
             let tls = BambuTls::resolve(entry.ca_cert_path.as_deref().map(Path::new), entry.require_valid_cert)?;
-            let printer = BambuPrinter::new(&entry.id, &entry.name, &host, &access_code, tls);
-            Ok(if entry.vendor == "bambu_v1" {
-                PrinterHandle::BambuV1(printer)
-            } else {
-                PrinterHandle::BambuV2(printer)
-            })
+            Ok(PrinterHandle::BambuV1(BambuV1Printer::new(&entry.id, &entry.name, &host, &access_code, &sn, tls)))
+        }
+        "bambu_v2" => {
+            let host = require_field(&entry.id, "host", entry.host)?;
+            let access_code = require_field(&entry.id, "access_code", entry.access_code)?;
+            let sn = require_field(&entry.id, "sn", entry.sn)?;
+            let tls = BambuTls::resolve(entry.ca_cert_path.as_deref().map(Path::new), entry.require_valid_cert)?;
+            Ok(PrinterHandle::BambuV2(BambuV2Printer::new(&entry.id, &entry.name, &host, &access_code, &sn, tls)))
         }
         "prusa_link" => {
             let host = require_field(&entry.id, "host", entry.host)?;