der: order FlagSet by its BIT STRING encoding - #2451

ValueOrd for FlagSet<T> compares bits() as an integer. The DER encoding, a NamedBitList BIT STRING, puts flag 0 in the most significant bit of the first octet and drops trailing zero bits, so the two orders disagree. For a 9-flag set:

{A}  bits 0x001  ->  03 02 07 80
{B}  bits 0x002  ->  03 02 06 40   <- first in DER

Over all 512×512 pairs of a 9-flag set, 84,822 pairs order differently.

X.690 11.6 orders SET OF components by their encodings. A SetOfVec of a type that contains a FlagSet can therefore encode out of DER order. An example is x509-cert's DistributionPoint, which derives ValueOrd and has reasons: Option<ReasonFlags>.

The fix compares the encoded values, using a 17-byte stack buffer: one unused-bits octet plus up to 16 data octets for u128 flags.

Test: bit_string::tests::flagset_valueord_is_encoding_order checks the {A}/{B} example and every pair of a 9-flag set against the order of their encodings. It fails on master.

This PR is independent of the other der ordering PRs (tag octet order, UtcTime, Option::der_cmp, ContextSpecific recursion) and merges cleanly with each.

This PR was produced by AI agents (Claude) while auditing every DerOrd/ValueOrd impl in der after differential fuzzing of der/x509-cert found the tag-order issue.