Fix /path crash on clear-terrain diffraction loss

TerrainAnalysis.deygout_diffraction/2 short-circuits to integer 0
when the path has no interior points or the principal edge nu is
below the shadow boundary, which then made Float.round/2 blow up
inside compute_loss_budget when the rest of the losses were all
floats. Multiply the diffraction value by 1.0 before it feeds into
the total so clear paths coerce cleanly without touching
TerrainAnalysis's (number()) contract.

lib/microwaveprop_web/live/path_live.ex

@@ -380,7 +380,9 @@ defmodule MicrowavepropWeb.PathLive do
 
     diffraction_loss =
       if terrain_result do
-        terrain_result.analysis.diffraction_db
+        # TerrainAnalysis.deygout_diffraction returns integer 0 for clear
+        # paths; coerce to float so Float.round/2 below is happy.
+        terrain_result.analysis.diffraction_db * 1.0
       else
         0.0
       end