Support direct Alkalinity-only input for fixed-pH screening

[TimBartholomew]

Description

This PR fixes alkalinity handling in build_water_composition(...) for fixed-pH screening workflows.

Previously, passing alkalinity directly was broken and could fail with a NameError due to an input_formula / input_fomrula typo. In addition, alkalinity-related inputs could trigger coupled carbonate handling that caused PHREEQC to reconcile carbonate chemistry and shift the specified pH.

This change makes the direct Alkalinity path explicit:

• Direct Alkalinity input now works without crashing.
• When Alkalinity is passed explicitly, the generated SOLUTION block includes only an Alkalinity line.
• The direct Alkalinity path does not auto-add C(4) or C(+4), which preserves the caller-specified pH in fixed-pH screening workflows.
• The existing CaHCO3 helper behavior is preserved as the explicit coupled carbon + alkalinity path.

Implementation

• Added a dedicated Alkalinity branch so direct alkalinity input no longer falls through the generic species handling.
• Fixed the input_formula / input_fomrula typo in the same area.
• Added focused regression tests covering:
  • direct Alkalinity input emits only Alkalinity
  • CaHCO3 still emits coupled inorganic carbon + alkalinity behavior

Validation

Ran:

• pytest src/phreeqcinwt/tests/test_solution_build.py -k "alkalinity or cahco3" -q
• pytest src/phreeqcinwt/tests/test_solution_build.py::TestBuildSolution -q

Results:

• Focused alkalinity tests: 2 passed
• TestBuildSolution: 20 passed

Behavior Summary

Case A: direct Alkalinity

• Writes Alkalinity
• Does not write C(4) or C(+4)
• No crash

Case B: CaHCO3

• Existing coupled behavior remains