Verifies App Store JWS payloads (StoreKit signed transactions and App Store Server Notifications V2).

The signing key comes from the leaf certificate in the JWS x5c header, but only after the full certificate chain is validated against the pinned Apple Root CA - G3 trust anchor. A self-signed or otherwise unchained certificate is rejected, so a payload cannot be forged by placing an attacker-controlled key in the header. If the pinned root is not installed, verification fails closed.

The pinned root lives at priv/certs/apple_root_ca_g3.pem — obtain it from https://www.apple.com/certificateauthority/AppleRootCA-G3.cer.