Guard against zero specific humidity in theta-e computation

lib/microwaveprop/weather/theta_e.ex

@@ -24,6 +24,10 @@ defmodule Microwaveprop.Weather.ThetaE do
   """
   @spec compute(float, float, float) :: float
   def compute(temp_k, spfh, pressure_pa) do
+    # Guard: zero or negative humidity makes the log terms undefined.
+    # Clamp to a tiny positive value (equivalent to extremely dry air).
+    spfh = max(spfh, 1.0e-8)
+
     # Mixing ratio from specific humidity: r = q / (1 - q)
     r = spfh / (1.0 - spfh)