Certs:
- Vendor Bambu's shared LAN-mode root CA (certs/bambu_ca2.pem, verified
self-signed CA:TRUE, see certs/README.md for provenance/fingerprint).
- Config gains bambu_ca_cert_path (per-printer override) and
bambu_require_valid_cert (the allow/reject flag); printer::bambu::BambuTls
turns those into BundledCa/Custom/Insecure. connect() doesn't perform a
real handshake yet (no TLS-capable MQTT client wired in), just reports
which trust mode it would use.
Also reconciles a rename in flight (Printer -> GenericPrinter trait) and
finishes the PrusaPrinter -> PrusaLinkPrinter/PrusaSerialPrinter split:
added the missing GenericPrinter impl for PrusaSerialPrinter, fixed
PrinterHandle's variant payload types, updated mod.rs's pub use list and
doc comments, and updated the example to the new 5-variant shape.
Verified with cargo check --all-targets (0 errors) and a full run of
cargo run --example printer_polymorphism.
