refactor(simplex): share variable transformation helpers

[rasheedja]

Summary

• Share shift and split coefficient-map substitution helpers between objectives and constraints, and place both helpers together after applySplitToConstraint.
• Use record field accessors and the existing field lenses to avoid repeated constructor branches and ambiguous lhs/rhs record updates.
• Preserve constraint relations, objective direction, shift offsets, and variable allocation.
• Export the helpers alongside the existing internal testing functions and add 15 direct tests, including two generated properties for algebraic substitution. Cover absent variables, empty maps, signed/fractional/zero coefficients, exact offsets, and preservation of unrelated terms.
• Update the changelog.

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

• Reproduced both original constraint-update warnings as build failures with -Werror=ambiguous-fields.
• stack --no-system-ghc test --no-terminal --force-dirty --ghc-options='-fforce-recomp -Werror=ambiguous-fields': 286 examples, 0 failures using GHC 9.6.7. Each new property ran 100 cases.
• cabal check, Fourmolu 0.15.0.0, and git diff --check passed.
• stack --no-system-ghc haddock --no-haddock-deps --no-terminal --ghc-options=-Werror=ambiguous-fields passed.