c1a839f5

By: ValentinHirschi <valentin.hirschi@gmail.com>

retain numerical CFF oracles in isolated Nix tests

Enable the existing eval feature in three-dimensional-reps Nix test contexts.
The isolated crate lacked the incoming feature supplied by ordinary workspace
builds, leaving its numerical oracle unavailable. Add the matching optional
feature guard to the one previously unguarded oracle test so bare no-eval
crate tests also compile. Corrected Nix and all-package test builds still
execute that unchanged numerical test and its assertions.

Independent feature-owner/body-equivalence review, Rust formatting and diff
checking pass. Remote compilation and the final local suites remain pending;
this is not the ALL_GREEN milestone. The running scalar checkout is unchanged.

User prompt:
Yes, continue as planned, it seems like it should be ok memory wise, so just let it run until LU scalar matrix is complete, and then really finalise the ALL_GREEN PR as mentioned earlier.

Related user instruction:
Don't let my need for approval stop you again. If you must take a decision that you think is within the scope of my approval, triple-check that you really need it (not like point 2. before) and then choose the answer for yourself, and just write it in "PENDING_APPROVALS.md" and continue. I'll periodically come and ask you about that file's content, but no longer wait for me, just make a decision and proceed towards the goal completion.