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/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
+        }
     }
 }