Simplify printer/ module for someone learning Rust

Drop reqwest, RPITIT+Send signature, and multi-variant error enum from the
teaching example so the only new idea per file is the trait override itself.
Real networking stays in adapters/, unaffected.

examples/printer_polymorphism.rs

@@ -4,7 +4,7 @@
 //! - `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
+//! - `dispatch_gcode()` 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.
 
@@ -12,33 +12,29 @@ use continuum_proxy::printer::{BambuPrinter, KlipperPrinter, Printer, PrinterHan
 
 #[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::Bambu(BambuPrinter::new("p1", "X1 Carbon", "192.168.1.50", "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")),
+        // No host configured — this one will hit the Err path.
+        PrinterHandle::Klipper(KlipperPrinter::new("p4", "Broken Voron", "")),
     ];
 
-    // 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.
+    // Uniform call site: every printer connects the same way from the
+    // caller's point of view, even though the three `connect()` bodies
+    // are unrelated implementations.
     for printer in &mut fleet {
         match printer.connect().await {
-            Ok(()) => println!("connected: {}", printer.display_name()),
-            Err(err) => println!("connect failed: {err}"),
+            Ok(()) => println!("  -> connected: {}\n", printer.display_name()),
+            Err(err) => println!("  -> failed: {err}\n"),
         }
     }
 
     // Vendor-only extension: reachable only after matching the concrete
     // variant back out of the enum.
-    for printer in &mut fleet {
+    for printer in &fleet {
         if let PrinterHandle::Bambu(bambu) = printer {
-            bambu
-                .dispatch_gcode_ftp(std::path::Path::new("./part.gcode.3mf"), "part.gcode.3mf")
-                .await
-                .ok();
+            bambu.dispatch_gcode("part.gcode.3mf");
         }
     }
 }