Add a caching mechanism

In the case where a request is made and a result has already been computed for the exact same inputs (checked by hashes) and the same server code (checked by hash also, or unique id generated by make install), return the cached file instead of redoing the computation.

The issue names no files or tests. Start by tracing the request-handling entry point and the existing result-file and hash logic, then inspect how make install identifies server code. Done means an exact input and server-code match returns the cached file without recomputing the result.

Written by the indexing model from the issue text.