Currently the damaging of input data for decryption (either ciphertext
or aad) only ever happens in the lower nibble within the byte at the
damage position, as the bit position for the `damage_val` byte was
calculated with `damage_bit & 3` (corresponding to `% 4`) rather than
`damage_bit & 7` (corresponding to the expected `% 8`).
follow-up to #28008.
* move `dummy_tag` variable in FSChaCha20Poly1305 crypto_tests
outside of the loop to be reused every time
* use easy to read `cipher.last()` in `AEADChaCha20Poly1305::Decrypt()`
* comment for initiator in `BIP324Cipher::Initialize()`
* systematically damage ciphertext with bit positions in bip324_tests
* use 4095 max bytes for aad in bip324 fuzz test
