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/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,
+        }
+    }
 }