refactor: make CodingRate a plain u8 denominator (5-8) on the wire

CodingRate was an enum with repr(u8) values that didn't match postcard's
wire encoding (variant index). Instead of papering over the mismatch,
make cr a plain u8 in RadioConfig. Host sends the denominator directly:
5 for CR 4/5, 6 for CR 4/6, etc. No lookup tables, no index confusion.

Remove CodingRate enum, cr_denom() helper, and the Bandwidth import that
was only needed for cr_denom. ErrorCode repr values also fixed to 0-based.

src/display/render.rs

@@ -106,7 +106,7 @@ pub fn dashboard(
                 Bandwidth::Khz250 => "250kHz",
                 Bandwidth::Khz500 => "500kHz",
             };
-            let _ = write!(buf, "{} SF{} CR4/{}", bw_str, cfg.sf, cr_denom(cfg.cr));
+            let _ = write!(buf, "{} SF{} CR4/{}", bw_str, cfg.sf, cfg.cr);
             Text::new(&buf, Point::new(0, FONT_H * 2 - 1), style)
                 .draw(target)
                 .ok();
@@ -244,12 +244,3 @@ pub fn blank(target: &mut impl DrawTarget<Color = BinaryColor>) {
     let _ = target.clear(BinaryColor::Off);
 }
 
-fn cr_denom(cr: crate::protocol::CodingRate) -> u8 {
-    use crate::protocol::CodingRate;
-    match cr {
-        CodingRate::Cr4_5 => 5,
-        CodingRate::Cr4_6 => 6,
-        CodingRate::Cr4_7 => 7,
-        CodingRate::Cr4_8 => 8,
-    }
-}

src/protocol.rs

@@ -20,23 +20,15 @@ pub enum Bandwidth {
     Khz500 = 9,
 }
 
-/// LoRa forward error correction coding rate.
-#[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, defmt::Format)]
-#[repr(u8)]
-pub enum CodingRate {
-    Cr4_5 = 1,
-    Cr4_6 = 2,
-    Cr4_7 = 3,
-    Cr4_8 = 4,
-}
-
 /// Complete LoRa radio configuration.
 #[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, defmt::Format)]
 pub struct RadioConfig {
     pub freq_hz: u32,
     pub bw: Bandwidth,
+    /// Spreading factor (5-12).
     pub sf: u8,
-    pub cr: CodingRate,
+    /// Coding rate denominator (5-8). E.g. 5 = CR 4/5, 8 = CR 4/8.
+    pub cr: u8,
     pub sync_word: u16,
     pub tx_power_dbm: i8,
 }
@@ -77,13 +69,15 @@ pub enum Response {
 }
 
 /// Error codes reported to the host.
+///
+/// Variant indices match postcard wire encoding (0-based).
 #[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, defmt::Format)]
 #[repr(u8)]
 pub enum ErrorCode {
-    InvalidConfig = 1,
-    RadioBusy = 2,
-    TxTimeout = 3,
-    CrcError = 4,
-    NotConfigured = 5,
-    NoDisplay = 6,
+    InvalidConfig = 0,
+    RadioBusy = 1,
+    TxTimeout = 2,
+    CrcError = 3,
+    NotConfigured = 4,
+    NoDisplay = 5,
 }

src/radio/task.rs

@@ -206,13 +206,17 @@ fn to_sf(sf: u8) -> lora_phy::mod_params::SpreadingFactor {
     }
 }
 
-fn to_cr(cr: protocol::CodingRate) -> lora_phy::mod_params::CodingRate {
+fn to_cr(cr: u8) -> lora_phy::mod_params::CodingRate {
     use lora_phy::mod_params::CodingRate::*;
     match cr {
-        protocol::CodingRate::Cr4_5 => _4_5,
-        protocol::CodingRate::Cr4_6 => _4_6,
-        protocol::CodingRate::Cr4_7 => _4_7,
-        protocol::CodingRate::Cr4_8 => _4_8,
+        5 => _4_5,
+        6 => _4_6,
+        7 => _4_7,
+        8 => _4_8,
+        _ => {
+            debug_assert!(false, "BUG: unvalidated CR {}", cr);
+            _4_5
+        }
     }
 }