Add GenericPrinter::set_fan_speed() across all five printer kinds

Same override pattern as connect(): required, no default body, five
completely different bodies.

- BambuGenericPrinter::set_fan_speed_impl() is shared by BambuV1Printer and
  BambuV2Printer (composition reuse, same as connect_impl) - both speak
  the same MQTT gcode-injection mechanism.
- PrusaLinkPrinter and PrusaSerialPrinter both send the same M106 gcode
  but over different transports (HTTP command injection vs. raw serial
  bytes) - they share the percent->PWM conversion despite having unrelated
  connect() implementations.
- KlipperPrinter uses a hypothetical Moonraker-native endpoint that takes
  a percentage directly - no PWM conversion at all, since that math only
  applies to the gcode-speaking vendors.

percent_to_pwm() lives in mod.rs as a private free function rather than a
trait default: it's genuine shared logic, but only for the subset of
vendors that need it, which is exactly the case a trait default can't
express cleanly. Visible to bambu.rs/prusa.rs via super:: because Rust's
module privacy reaches into child modules.

Verified with cargo check --all-targets and a full run of
printer_polymorphism: 50% converges on the same PWM value (127) across all
three gcode-based printers, Klipper's native path takes 0.50 directly.

src/printer/klipper.rs

@@ -28,4 +28,13 @@ impl GenericPrinter for KlipperPrinter {
         println!("[{}] connecting over WebSocket (Moonraker)", self.base.name);
         Ok(())
     }
+
+    /// OVERRIDE: Moonraker's own **hypothetical** native API, not gcode injection — no
+    /// percent-to-PWM conversion needed since it already takes a
+    /// percentage directly. Real version:
+    /// `POST /printer/gcode/script?script=SET_FAN_SPEED SPEED=0.NN`.
+    async fn set_fan_speed(&mut self, percent: u8) -> Result<(), PrinterError> {
+        println!("[{}] Moonraker: SET_FAN_SPEED SPEED={:.2}", self.base.name, percent as f32 / 100.0);
+        Ok(())
+    }
 }