check signed-message length in AIMerSigner.verifySignature

[rootvector2]

AIMerSigner.verifySignature slices the signature out of the message || signature envelope at offset message.length without checking the buffer is that long, so a truncated signature throws ArrayIndexOutOfBoundsException out of Signature.verify() and a valid one with bytes appended still verifies (found by feeding wrong-length signatures to every BCPQC Signature service); it now requires exactly message.length + getSignatureBytes() and returns false otherwise, the guard the Falcon, Faest, Mayo, Snova and QRUOV signers already carry.