Add Printer trait + PrinterHandle enum domain model with polymorphism example

README.md

@@ -31,7 +31,21 @@ cargo run
 src/
   main.rs            Multi-threaded async engine: uplink, discovery, go2rtc watchdog
   uplink/             Cloud WebSocket client (reconnect, heartbeat, dispatch)
-  adapters/           bambu.rs, prusalink.rs, moonraker.rs printer clients
+  adapters/           bambu.rs, prusalink.rs, moonraker.rs — wire-protocol clients
+  printer/            Domain model: Printer trait + PrinterBase + PrinterHandle enum
   plate_changer/      Mechanical cycle control + sensor validation
   discovery/          LAN printer discovery (SSDP / mDNS / static config)
 ```
+
+`src/adapters/` talks the raw wire protocols (MQTT, FTPS, HTTP, WS).
+`src/printer/` is the vendor-agnostic domain model layered on top: a
+`Printer` trait plus one struct per vendor (`BambuPrinter`, `PrusaPrinter`,
+`KlipperPrinter`) wrapped in a closed `PrinterHandle` enum. Rust has no class
+inheritance, so this trait+enum combo is what stands in for a
+`GenericPrinter -> BambuPrinter` hierarchy — composition for shared fields,
+a trait for shared/overridable behavior, an enum for the closed set of
+vendors. See `examples/printer_polymorphism.rs`:
+
+```bash
+cargo run --example printer_polymorphism
+```