Update docs and prune examples superseded by real functionality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  fully superseded by printers.toml's per-printer ca_cert_path/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

.env.example

@@ -10,11 +10,10 @@ CONTINUUM_UPLINK_URL=wss://api.continuum.local/ws/edge/v1
 CONTINUUM_GO2RTC_BIN=/usr/local/bin/go2rtc
 CONTINUUM_GO2RTC_CONFIG=./go2rtc.yaml
 
-# Bambu MQTTS certificate trust. Most printers verify fine against the
-# bundled shared CA (certs/bambu_ca2.pem) and need nothing here. For a
-# printer that doesn't chain to it (P1P, at least), add "printer_id=path"
-# entries, comma-separated: CONTINUUM_BAMBU_CA_CERT_OVERRIDES=p1=./certs/p1p.pem
-CONTINUUM_BAMBU_CA_CERT_OVERRIDES=
-CONTINUUM_BAMBU_REQUIRE_VALID_CERT=true
+# Fleet config - see printers.example.toml. Bambu MQTTS certificate trust
+# (per-printer ca_cert_path/require_valid_cert, and automatic
+# trust-on-first-connect pinning otherwise) is configured per printer in
+# that file, not here.
+CONTINUUM_PRINTERS_PATH=printers.toml
 
 RUST_LOG=info,continuum_proxy=debug