Bambu TLS trust config + finish PrusaLink/PrusaSerial split

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.

src/printer/klipper.rs

@@ -1,4 +1,4 @@
-use super::{Printer, PrinterBase, PrinterError};
+use super::{GenericPrinter, PrinterBase, PrinterError};
 
 pub struct KlipperPrinter {
     base: PrinterBase,
@@ -12,7 +12,7 @@ impl KlipperPrinter {
     }
 }
 
-impl Printer for KlipperPrinter {
+impl GenericPrinter for KlipperPrinter {
     fn base(&self) -> &PrinterBase {
         &self.base
     }