fix(sign): match cloud signature to requested digest

[Loggableim]

What changed

• Select the cloud signature by the exact digest requested from Certum.
• Reject a response that lacks that digest instead of taking the first map entry.
• Add regression tests for multi-entry responses and missing requested digests.

Scope

This PR intentionally contains only the response-selection fix. Timestamp transport policy, Content-Type handling, PE bounds checks, and output-write hardening are excluded.

[Le-Syl21]

Thanks — the selection bug is real: values().next() can pick a signature that doesn't match the digest we asked for, so keying on the requested digest is the right call.

One thing before this goes in. Map::get is an exact, case-sensitive match, and we build the key with hex::encode, which is lowercase. Even if Certum happens to echo lowercase today, that isn't something we can rely on: what the protocol gives us — in the docs and by the nature of the value — is hex. The case is not part of that contract, so it can change without Certum breaking anything on their side, and observing it once wouldn't make it a guarantee. The failure mode is every signing run turning into no signature for requested digest.

Since the case carries no information for a hex string, I'd rather not depend on it:

.and_then(|m| {
    m.iter()
        .find(|(k, _)| k.eq_ignore_ascii_case(digest_hex))
        .map(|(_, v)| v)
})

Same guarantee, no allocation, and nothing to break on a formatting detail that was never promised. Worth adding a test with a realistic 64-char digest in both cases too — the current ones use "wanted"/"other", which don't exercise that path.