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.

examples/printer_polymorphism.rs

@@ -62,4 +62,13 @@ async fn main() {
             _ => {}
         }
     }
+
+    // A second trait method, same uniform call site as connect(): every
+    // vendor sends "50%" a completely different way — an MQTT gcode
+    // command, Moonraker's native API, gcode over HTTP, gcode over serial
+    // — but the caller doesn't need to know or care which.
+    println!();
+    for printer in &mut fleet {
+        let _ = printer.set_fan_speed(50).await;
+    }
 }

src/printer/bambu.rs

@@ -252,6 +252,17 @@ impl BambuGenericPrinter {
         }
     }
 
+    /// Shared by both `BambuV1Printer` and `BambuV2Printer` — same MQTT
+    /// gcode-injection mechanism regardless of protocol generation. Real
+    /// version would publish
+    /// `{"print":{"command":"gcode_line","param":"M106 P1 S<pwm>"}}` to
+    /// `device/{sn}/request` over the MQTTS session.
+    async fn set_fan_speed_impl(&mut self, percent: u8) -> Result<(), PrinterError> {
+        let pwm = super::percent_to_pwm(percent);
+        println!("[{}] sending gcode: M106 P1 S{pwm}", self.base.name);
+        Ok(())
+    }
+
     /// Shared "connect" logic both V1 and V2 delegate to — the real
     /// version would open the MQTTS session here. Still a stub, but no
     /// longer duplicated across two structs.
@@ -302,6 +313,10 @@ impl GenericPrinter for BambuV1Printer {
     async fn connect(&mut self) -> Result<(), PrinterError> {
         self.generic.connect_impl().await
     }
+
+    async fn set_fan_speed(&mut self, percent: u8) -> Result<(), PrinterError> {
+        self.generic.set_fan_speed_impl(percent).await
+    }
 }
 
 /// Current-generation Bambu MQTT report schema (X1 Carbon/X1E, H2 series).
@@ -341,4 +356,8 @@ impl GenericPrinter for BambuV2Printer {
     async fn connect(&mut self) -> Result<(), PrinterError> {
         self.generic.connect_impl().await
     }
+
+    async fn set_fan_speed(&mut self, percent: u8) -> Result<(), PrinterError> {
+        self.generic.set_fan_speed_impl(percent).await
+    }
 }

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(())
+    }
 }

src/printer/mod.rs

@@ -67,6 +67,12 @@ pub trait GenericPrinter {
     /// into the multi-threaded daemon — safe to ignore for now.
     async fn connect(&mut self) -> Result<(), PrinterError>;
 
+    /// REQUIRED, no default body: `percent` is 0-100, but how it actually
+    /// gets sent — an MQTT gcode-injection command, a REST call, raw bytes
+    /// on a serial port — is different for every vendor, same as
+    /// `connect()`.
+    async fn set_fan_speed(&mut self, percent: u8) -> Result<(), PrinterError>;
+
     /// OPTIONAL: a default body every printer gets for free unless it
     /// writes its own. None of ours do below, so all three vendors share
     /// this exact implementation — the trait-method equivalent of
@@ -76,14 +82,17 @@ pub trait GenericPrinter {
     }
 }
 
-pub enum PrinterFlavour {
-    BambuV1Printer,
-    BambuV2Printer,
-    PrusaLinkPrinter,
-    PrusaSerialPrinter,
-    KlipperPrinter,
+/// M106's `S` parameter is a raw 0-255 PWM value, not a percentage. Shared
+/// by anything that ultimately sends gcode (Bambu, PrusaLink, PrusaSerial)
+/// — *not* a trait default, since Klipper's hypothetical native fan API
+/// below takes a percent directly and has no use for this at all. Private,
+/// but still visible to bambu.rs/prusa.rs: Rust's module privacy reaches
+/// into child modules, not just the exact file it's defined in.
+fn percent_to_pwm(percent: u8) -> u8 {
+    ((percent.min(100) as u16 * 255) / 100) as u8
 }
 
+
 /// The closed set of printer kinds this fleet can talk to.
 pub enum PrinterHandle {
     BambuV1(BambuV1Printer),
@@ -117,4 +126,14 @@ impl GenericPrinter for PrinterHandle {
             PrinterHandle::Klipper(p) => p.connect().await,
         }
     }
+
+    async fn set_fan_speed(&mut self, percent: u8) -> Result<(), PrinterError> {
+        match self {
+            PrinterHandle::BambuV1(p) => p.set_fan_speed(percent).await,
+            PrinterHandle::BambuV2(p) => p.set_fan_speed(percent).await,
+            PrinterHandle::PrusaLink(p) => p.set_fan_speed(percent).await,
+            PrinterHandle::PrusaSerial(p) => p.set_fan_speed(percent).await,
+            PrinterHandle::Klipper(p) => p.set_fan_speed(percent).await,
+        }
+    }
 }

src/printer/prusa.rs

@@ -28,6 +28,15 @@ impl GenericPrinter for PrusaLinkPrinter {
         println!("[{}] connecting over HTTP (PrusaLink)", self.base.name);
         Ok(())
     }
+
+    /// OVERRIDE: gcode via PrusaLink's command-injection endpoint — same
+    /// underlying M106 as Bambu, reusing the shared percent->PWM
+    /// conversion, but sent over HTTP instead of MQTT.
+    async fn set_fan_speed(&mut self, percent: u8) -> Result<(), PrinterError> {
+        let pwm = super::percent_to_pwm(percent);
+        println!("[{}] POST /api/printer/command {{\"commands\":[\"M106 S{pwm}\"]}}", self.base.name);
+        Ok(())
+    }
 }
 
 
@@ -60,4 +69,13 @@ impl GenericPrinter for PrusaSerialPrinter {
         println!("[{}] connecting over serial (COM{})", self.base.name, self.com_port);
         Ok(())
     }
+
+    /// OVERRIDE: same gcode as PrusaLink (still M106, still the shared
+    /// conversion), but written straight to the serial port instead of
+    /// sent over HTTP — a third transport for the same underlying command.
+    async fn set_fan_speed(&mut self, percent: u8) -> Result<(), PrinterError> {
+        let pwm = super::percent_to_pwm(percent);
+        println!("[{}] writing to COM{}: M106 S{pwm}", self.base.name, self.com_port);
+        Ok(())
+    }
 }