Restore the psychrometric enthalpy axis

[iguchi-lab]

What changed

• restore the diagonal enthalpy axis from the Python psychrometric chart
• add the enthalpy-axis title and 0–120 kJ/kg(DA) scale labels
• draw short blue scale segments where enthalpy lines cross the axis
• limit the main enthalpy lines to the same side of the axis as the source implementation
• add regression assertions for the axis, title, and scale labels

Why

The initial browser SVG port included enthalpy grid lines but omitted the diagonal enthalpy scale axis from iguchi-lab/psychrometric-chart.

Validation

GitHub Actions runs the existing Python calculator suite and the browser chart rendering tests.