in the transverse update we recomputed density every passive iteration

[zingale]

this pulls that out.
A later optimization is to precompute the inverse, but that introduces roundoff diffs, so I'll save that for a follow-on PR

PR summary

PR motivation

PR checklist

• test suite needs to be run on this PR
• this PR will change answers in the test suite to more than roundoff level
• all newly-added functions have docstrings as per the coding conventions
• the CHANGES file has been updated, if appropriate
• if appropriate, this change is described in the docs

[zingale]

this seems to perhaps slow things down?

http://groot.astro.sunysb.edu/Castro/test-suite/gfortran/2025-12-17-001/index.html

[zingale]

tests pass now:
http://groot.astro.sunysb.edu/Castro/test-suite/gfortran/2025-12-17-003/index.html

I'll do a follow-on PR introducing the 1/rho temporary, which causes roundoff diffs

[zingale]

tests:
http://groot.astro.sunysb.edu/Castro/test-suite/gfortran/2025-12-22-001/index.html