Update docs and prune examples superseded by real functionality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  fully superseded by printers.toml's per-printer ca_cert_path/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

printers.example.toml

@@ -11,11 +11,12 @@ vendor = "bambu_v1"
 host = "192.168.1.50"
 access_code = "REPLACE_WITH_ACCESS_CODE"
 sn = "REPLACE_WITH_SERIAL_NUMBER"      # printer screen > Settings > Device (used for MQTT topics later)
-# P1P doesn't chain to the bundled CA - point this at a cert you've
-# downloaded from the printer itself. Leave unset to use the bundled CA.
-# `cargo run --example fetch_bambu_cert -- p1p ./certs/p1p.pem` can fetch it
-# for you (trust-on-first-connect - see that example's doc comment).
-ca_cert_path = "./certs/p1p.pem"
+# No ca_cert_path needed even though P1P doesn't chain to the bundled CA -
+# `connect()` auto-pins a certificate via trust-on-first-connect the first
+# time it can't verify against the bundled one, and saves it under
+# certs/pinned/<sn>.pem for every run after that (see BambuPrinter::
+# ensure_trusted's doc comment). Only set ca_cert_path explicitly if you
+# want to point at a certificate you've captured/vetted some other way.
 
 [[printer]]
 id = "h2c"