der: fix infinite recursion in ValueOrd for ContextSpecific - #2450

ValueOrd for ContextSpecific<T> recurses into itself forever. The same impl, generated by the same macro, is used for Application<T> and Private<T>:

fn value_cmp(&self, other: &Self) -> Result<Ordering, Error> {
    match self.tag_mode {
        TagMode::Explicit => self.der_cmp(other),   // der_cmp -> headers equal -> self.value_cmp
        TagMode::Implicit => self.value_cmp(other), // calls itself
    }
}

Comparing two fields with the same header overflows the stack:

let x = ContextSpecific { tag_number: TagNumber(1), tag_mode: TagMode::Implicit, value: 5u8 };
let y = ContextSpecific { tag_number: TagNumber(1), tag_mode: TagMode::Implicit, value: 6u8 };
x.der_cmp(&y); // thread 'main' has overflowed its stack

Sorting a SetOfVec<ContextSpecific<_>> hits this too.

The fix compares the contents octets: self.value.der_cmp(&other.value) (the inner TLV) for EXPLICIT, and self.value.value_cmp(&other.value) for IMPLICIT.

The bug goes back to the original *Ord traits (#190) and was carried into the class macro in #1819.

Test: context_specific::tests::value_ord covers both tag modes with value_cmp/der_cmp. On master it aborts with a stack overflow.

This PR is independent of the three SET OF ordering PRs (tag octet order, UtcTime, Option::der_cmp) 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.