This is a deliberately dormant follow-up PR for the Staircase SEC processing and optical fitting work split out of #197.

Do not review or merge this yet. It is opened now only to preserve the work and make the intended follow-up scope visible.

Dependency

This PR depends on #197. Until #197 is merged, this PR will show a lot of overlapping EC-optical reader/measurement changes from the main PR. That is expected.

After #197 lands, this branch should be rebased or otherwise updated onto the merged main branch so the diff is reduced to only the extension work:

• StaircaseSEC
• StaircaseSECPlotter
• OpticalCVFitting
• the staircase extraction/demo workflow

If #197 is merged with the same commits, this should become straightforward. If #197 is squash-merged, we should rebase this branch onto the new upstream main and replay/cherry-pick the WIP extension commit.

Follow-up questions for this PR

The main design work still needed here is:

• Decide whether staircase extraction should become an ECOpticalMeasurement method, an ixdat Calculator, or another structured object.
• Rework OpticalCVFitting so it follows ixdat's calculator pattern and can round-trip cleanly through as_dict() / from_dict().
• Replace hard-coded BioLogic column names in OpticalCVFitting with ixdat aliases or configurable series names.
• Add proper docs for the processed SEC and fitting workflow.

The point of splitting this out is to let #197 focus on the core EC-optical measurement/reader functionality while this more experimental processing and fitting work can continue in the background.