refactor(simplex): use standard map selection operations

[rasheedja]

Summary

• Replace membership-based map filtering with Data.Map.restrictKeys and Data.Map.withoutKeys from the existing containers dependency.
• Build artificial-variable sets once per phase-one operation and use Data.Map.delete for the entering variable.
• Remove the unused negatedRows binding next to the updated artificial-objective selection. Preserve variable ordering and coefficients.

This cleanup targets master independently; it does not require the correctness fixes to merge first. The map-fold and printer cleanups are already included in #18 and #19.

Validation

• stack test --no-terminal: 271 examples, 0 failures on this branch.
• cabal check, Fourmolu 0.15.0.0, git diff --check, and stack haddock --no-haddock-deps passed.
• All five cleanup branches merge cleanly with each other and fixes fix(simplex): handle phase-one variables and constant constraints #17–fix(prettify): render fractions and separators correctly #19 in a temporary integration worktree.
• Combined integration suite: 288 examples, 0 failures.