fix(simplex): handle phase-one variables and constant constraints

[rasheedja]

Summary

• Reserve variables from every transformed objective before allocating slack, artificial, and objective variables. Previously, maximizing x1 + x2 with nonnegative variables and only x1 <= 1 could alias x2 with a slack variable and incorrectly report a finite optimum instead of Unbounded.
• Evaluate constant constraints before constructing phase one: discard tautologies and return infeasible for contradictions. This covers empty coefficient maps and maps containing only zero coefficients, avoiding the previous maximum [] exception.
• Keep the public API unchanged and add regression coverage for later objectives, transformed variables, and all three constraint relations.

These two review findings share phase-one input handling, so they are bundled to keep the PRs independently mergeable. This PR targets master and has no dependency on the other fixes.

Validation

• New regression tests reproduced failures before the fix.
• stack test --no-terminal: 285 examples, 0 failures.
• cabal check: no errors or warnings.
• Fourmolu 0.15.0.0 check of all tracked Haskell files passed.
• stack haddock --no-haddock-deps and git diff --check passed.