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.
