ctap2: Set repr(u8) for Error and implement From<Error> for u8

Fixes: https://github.com/trussed-dev/ctap-types/issues/63

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Remove unused `Rpc` trait.
 - Remove `cbor-smol` re-export as `serde`.
 - Make `ctap2::CtapMappingError` private.
+- Set `#[repr(u8)]` for `ctap2::Error` and implement `From<ctap2::Error>` for `u8`.
 
 ## [0.6.0-rc.4] 2026-06-01
 

src/ctap2.rs

@@ -444,6 +444,7 @@ impl<'de> Deserialize<'de> for AttestationFormatsPreference {
 
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 #[non_exhaustive]
+#[repr(u8)]
 pub enum Error {
     Success = 0x00,
     InvalidCommand = 0x01,
@@ -502,6 +503,12 @@ pub enum Error {
     VendorLast = 0xFF,
 }
 
+impl From<Error> for u8 {
+    fn from(error: Error) -> u8 {
+        error as _
+    }
+}
+
 /// CTAP2 authenticator API
 pub trait Authenticator {
     fn get_info(&mut self) -> get_info::Response;