Update docs and prune examples superseded by real functionality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  fully superseded by printers.toml's per-printer ca_cert_path/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

certs/README.md

@@ -28,3 +28,16 @@ certificates at the same time.
 If a printer's connection ever fails certificate verification against this
 file, re-fetch from the source URL above (BambuStudio ships whatever the
 current fleet needs) and update the fingerprint here.
+
+## `pinned/`
+
+Not checked in (gitignored) - where `BambuPrinter::ensure_trusted` saves a
+certificate it auto-pinned via trust-on-first-connect, one file per
+printer, named `<serial number>.pem`. This only happens for a printer
+whose certificate doesn't verify against `bambu_ca2.pem` above (P1P, at
+least) and that doesn't already have an explicit `ca_cert_path` set in
+`printers.toml` - see that method's doc comment in `src/printer/bambu.rs`
+for the full trust-on-first-connect flow, and the SSH-host-key analogy it's
+built on. Delete a printer's file here to force it to re-pin (a real
+error, not silent, if the certificate it captures the second time still
+doesn't verify) the next time it connects.