Add Printer trait + PrinterHandle enum domain model with polymorphism example

examples/printer_polymorphism.rs

@@ -0,0 +1,44 @@
+//! Run with: cargo run --example printer_polymorphism
+//!
+//! Demonstrates the trait+enum pattern that stands in for inheritance:
+//! - `connect()` is called the same way regardless of vendor (polymorphism),
+//!   but each vendor's `impl Printer` runs completely different code
+//!   (override).
+//! - `dispatch_gcode_ftp()` only exists on `BambuPrinter` (extension) — you
+//!   have to `match` the enum back down to the concrete type to reach it,
+//!   which is the trade-off for not having implicit downcasting.
+
+use continuum_proxy::printer::{BambuPrinter, KlipperPrinter, Printer, PrinterHandle, PrusaPrinter};
+
+#[tokio::main]
+async fn main() {
+    tracing_subscriber::fmt::init();
+
+    let mut fleet: Vec<PrinterHandle> = vec![
+        PrinterHandle::Bambu(BambuPrinter::new("p1", "X1 Carbon", "192.168.1.50:8883", "12345678")),
+        PrinterHandle::Prusa(PrusaPrinter::new("p2", "MK4", "192.168.1.51", "prusa-api-key")),
+        PrinterHandle::Klipper(KlipperPrinter::new("p3", "Voron 2.4", "192.168.1.52")),
+    ];
+
+    // Uniform call site: every printer is connected the same way from the
+    // caller's point of view, even though BambuPrinter::connect,
+    // PrusaPrinter::connect and KlipperPrinter::connect are three unrelated
+    // implementations.
+    for printer in &mut fleet {
+        match printer.connect().await {
+            Ok(()) => println!("connected: {}", printer.display_name()),
+            Err(err) => println!("connect failed: {err}"),
+        }
+    }
+
+    // Vendor-only extension: reachable only after matching the concrete
+    // variant back out of the enum.
+    for printer in &mut fleet {
+        if let PrinterHandle::Bambu(bambu) = printer {
+            bambu
+                .dispatch_gcode_ftp(std::path::Path::new("./part.gcode.3mf"), "part.gcode.3mf")
+                .await
+                .ok();
+        }
+    }
+}