Function verify_changeset_signature 

pub fn verify_changeset_signature(
    env: &ChangesetEnvelope,
    changeset_bytes: &[u8],
) -> bool

Verify the signature on a changeset envelope.

Returns true if:

• No signature is present (unsigned changesets are accepted).
• A valid signature is present that matches the author’s public key.

Returns false if a signature is present but invalid (wrong key, tampered data, or malformed hex).