Make Candle's compute_init set a boolean reference

At the moment, Candle's `compute_init` checks the characteristic equations but the fact that they were checked is not remembered. As a result, each call to `compute` checks the characteristic equations again, which would be unnecessary if Candle remembered (using a boolean reference) that the characteristic equations have been checked before.

This issue is about adding a boolean reference and offloading the checking of characteristic equations from `compute`.

Start with Candle's compute_init and compute entry points, which are the only locations named in the issue. Trace how characteristic-equation checking is currently triggered, then verify that compute_init records the completed check and later compute calls reuse that state without checking again.

Written by the indexing model from the issue text.