Cache classes which have already been verified as immutable.

Verifying immutability is recursive. In case of type composition, very often the same types will need to be verified.

Therefore, a straightforward performance improvement is to cache the outcome of classes that have already been verified as immutable (or not).

Start by locating the recursive immutability verification entry point and tracing where composed types are checked repeatedly. Add caching for both immutable and non-immutable outcomes, then verify that repeated type checks reuse the result without changing verification behavior.

Written by the indexing model from the issue text.