fix(attestation): reject oversized challenges and rewrite cert DER encoding

DuckDetector flagged two issues:
1. Oversized challenge accepted — 256-byte attestation challenge should
   return INVALID_INPUT_LENGTH (-21) like real KeyMint. Added early check
   in handleGenerateKey before any path decision.
2. Issuer/subject chain mismatch — rcgen's HashMap loses DN attribute
   ordering and converts PrintableString to UTF8String, producing
   different DER bytes. Replaced rcgen with manual DER assembly that
   injects raw keybox issuer_dn_der bytes directly.

Verified on device: TX_ID 315 rejects 256-byte challenge, TX_ID 501
generates valid 4-cert chain with correct issuer linkage.

native-certgen/src/error.rs

@@ -11,7 +11,6 @@ pub enum CertGenError {
     KeyboxParseFailed(String),
     AttestationBuildFailed(String),
     DerError(der::Error),
-    RcgenError(rcgen::Error),
     EmptyKeyboxChain,
     ChallengeTooLong(usize),
     InvalidParameter(String),
@@ -31,7 +30,6 @@ impl fmt::Display for CertGenError {
             Self::KeyboxParseFailed(msg) => write!(f, "keybox parse failed: {}", msg),
             Self::AttestationBuildFailed(msg) => write!(f, "attestation build failed: {}", msg),
             Self::DerError(e) => write!(f, "DER error: {}", e),
-            Self::RcgenError(e) => write!(f, "rcgen error: {}", e),
             Self::EmptyKeyboxChain => write!(f, "keybox certificate chain is empty"),
             Self::ChallengeTooLong(len) => write!(f, "attestation challenge too long: {} bytes (max 128)", len),
             Self::InvalidParameter(msg) => write!(f, "invalid parameter: {}", msg),
@@ -73,10 +71,5 @@ impl From<rsa::Error> for CertGenError {
     }
 }
 
-impl From<rcgen::Error> for CertGenError {
-    fn from(e: rcgen::Error) -> Self {
-        Self::RcgenError(e)
-    }
-}
 
 pub type Result<T> = std::result::Result<T, CertGenError>;