Consider using stronger parameter identification method

Currently, HIT just uses ciphertext level to identify compatible ciphertexts. However, this isn't really enough: ciphertexts have to be with respect to the same parameters (like moduli) and keys to be compatible. SEAL captures this notion with `params_id`. We'd like to avoid being coupled tightly to SEAL if possible, but it might also be nice to have additional safety checks in place.

Start by reviewing the current ciphertext-level compatibility check and how SEAL's params_id captures parameter identity. Determine an approach that checks matching parameters and keys without tightly coupling HIT to SEAL; done means the compatibility rules and safety checks are agreed and implemented.

Written by the indexing model from the issue text.