Use `ExceptT` instead of `State Bool` in the case-split plugin implementation

This is an implementation detail, but @MangoIV says it _could_ improve performance (I suppose in large ASTs?).

Worst case scenario, we don't lose anything.

As regards readability, I don't know.

Anyway, worth experimenting with this in spare time.

Originally considered during [my GSoC 2026 project](https://gist.github.com/Aster89/69a57c364ec2d0cc89f474cc3d194371#future-work--stretch-goals).

Start by locating the case-split plugin implementation and its use of State Bool. Compare an ExceptT-based experiment against the existing behavior, then establish whether large-AST performance improves without regressions; the issue does not name files or tests.

Written by the indexing model from the issue text.