Update docs and prune examples superseded by real functionality

README/certs/README/.env.example/printers.example.toml were all still
describing this as a learning-stage stub with no real printer protocol
clients - rewritten to reflect the real Bambu MQTT client, cert
auto-pinning, and marlin serial connection that now exist.

- Remove examples/fetch_bambu_cert.rs: its manual fetch-and-save workflow
  is now what BambuPrinter::connect()'s ensure_trusted does automatically
  on every connect, so it has no remaining use case.
- Fix examples/printer_polymorphism.rs, which no longer compiled -
  dispatch_gcode was replaced by real methods earlier this session;
  swapped in print_status() as the vendor-only-extension example.
- Remove Config::bambu_ca_cert_overrides/bambu_require_valid_cert and the
  .env vars documenting them - confirmed dead (nothing read them),
  fully superseded by printers.toml's per-printer ca_cert_path/
  require_valid_cert plus automatic trust-on-first-connect pinning.
- Fix stale doc comments in printer/mod.rs and printer/bambu.rs that
  described certificate trust and MQTT as not built yet, or referencing
  the now-removed example.

No behavior change in the daemon itself - verified with cargo build
--examples and a live run of printer_polymorphism and auto_connect_bambu
against the real P1P/H2C printers.

examples/printer_polymorphism.rs

@@ -56,9 +56,13 @@ async fn main() {
     // Vendor-only extension: reachable only after matching the concrete
     // variant back out of the enum - one arm now that Bambu is a single
     // type regardless of which protocol version `connect()` settled on.
+    // `print_status()` is real Bambu-only behavior with no equivalent on
+    // `GenericPrinter` (no other vendor here has an in-memory report
+    // cache) - none of these connected above (fake IPs), so it's always
+    // `None`, but the point is the downcast, not a live reading.
     for printer in &fleet {
         match printer {
-            PrinterHandle::Bambu(bambu) => bambu.dispatch_gcode("part.gcode.3mf"),
+            PrinterHandle::Bambu(bambu) => println!("  {}: cached print_status = {:?}", bambu.sn, bambu.print_status().await),
             _ => {}
         }
     }