fix: mask cert serial MSB to prevent DER Overlength panic in --generate

[ryancee]

Summary

--generate panics with Overlength at builder.rs:127 on approximately every other invocation.

Root cause

builder.rs generates a 20-byte random certificate serial and passes it directly to SerialNumber::new:

let mut serial = [0; 20];
OsRng.fill_bytes(&mut serial);
SerialNumber::new(&serial).expect("valid")   // panics ~50% of the time

A DER INTEGER requires a 0x00 sign-extension byte when the MSB of the first content byte is set (to indicate a positive value). For a 20-byte random array this happens ~50% of the time, producing a 21-byte encoding that exceeds the RFC 5280 §4.1.2.2 limit of 20 bytes. x509-cert 0.2 / der 0.7 returns Error { kind: Overlength, position: None } and the .expect("valid") call panics with:

thread 'main' panicked at src/builder.rs:127:40:
valid: Error { kind: Overlength, position: None }

Fix

Mask the high bit of the first byte before constructing SerialNumber to ensure the value is always non-negative and the DER encoding never exceeds 20 bytes:

serial[0] &= 0x7f;

This is the standard approach used by most X.509 certificate generators (see e.g. OpenSSL, rcgen, rustls-cert-gen). The existing TODO comment notes that SerialNumber::generate (RustCrypto/formats#1270) will eventually handle this correctly; until that lands, the manual mask is the correct fix.

Testing

Verified on YubiKey 5C NFC (serial 37153875, firmware 5.7.4) with AES256 management key. Previously --generate failed consistently on this device (panicking at builder.rs:127). With this fix applied the command completes successfully and produces a valid recipient/identity stub.

Also related to / builds on top of #219 (wrong OID in util.rs).

Note on branch base

This branch is based on the PR #190 branch (str4d:yubikey-0.8) which has not yet been merged. The fix applies equally to main once #190 merges — serial[0] &= 0x7f is a one-line change independent of the rest of the #190 changes.