feat(native-certgen): implement keybox DER certificate chain parser

Splits concatenated DER cert chains into individual certificates,
extracts leaf issuer DN and notAfter via x509-cert crate. Handles
multi-byte DER length encoding (0x81-0x84).

native-certgen/src/lib.rs

@@ -2,6 +2,6 @@ mod error;
 mod types;
 mod keygen;
 // mod attestation;  // Phase 2
-// mod keybox;       // Phase 0-1 Task 2
+pub mod keybox;
 // mod certbuilder;  // Phase 3
 // mod logging;      // Phase 3