Update docs and prune examples superseded by real functionality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  fully superseded by printers.toml's per-printer ca_cert_path/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/auto_connect_bambu.rs

@@ -1,12 +1,13 @@
 //! Run with: cargo run --example auto_connect_bambu
 //!
-//! The "smooth, no manual steps" version of test_bambu_certs +
-//! fetch_bambu_cert combined: for each Bambu printer in printers.toml,
-//! makes sure it has a working certificate, auto-pinning one via
-//! trust-on-first-connect if the bundled CA doesn't verify. Safe to run
-//! repeatedly - a printer that's already pinned (certs/pinned/<serial
-//! number>.pem) just gets re-verified against its pin, no network trust
-//! decision is made again.
+//! For each Bambu printer in printers.toml, makes sure it has a working
+//! certificate, auto-pinning one via trust-on-first-connect if the
+//! bundled CA doesn't verify - the exact same `ensure_trusted` step
+//! `BambuPrinter::connect()` runs before opening MQTT, isolated here so
+//! you can check/establish trust without needing a full MQTT handshake to
+//! succeed too. Safe to run repeatedly - a printer that's already pinned
+//! (certs/pinned/<serial number>.pem) just gets re-verified against its
+//! pin, no network trust decision is made again.
 
 use continuum_proxy::fleet;
 use continuum_proxy::printer::PrinterHandle;