Verifies Linked Data Signatures of the given JSON-LD document.

This is a low-level utility that only checks the cryptographic signature and (optionally) the cached key. It does not run the JSON-LD parsing, attribution, and owner checks that a complete inbound LD verification needs. For incoming activities, prefer verifyJsonLd, which is the public verification entry point and the one that emits the activitypub.signature.verification.duration metric for the LD path. verifySignature itself only emits activitypub.signature.key_fetch.duration, since the rest of the work that the verification-duration metric is meant to cover happens in verifyJsonLd.