fix(path): stop warning on expected native-duct cache misses

`Weather.nearest_native_duct_info/3` returns `{:error, :not_found}`
whenever the midpoint has no native HRRR profile within ±0.07° / ±1 h
— the documented fallback for the sparsely-populated
`hrrr_native_profiles` table (only QSO-worker-touched cells get
ingested). The empty-map fallback was already the right behavior; the
warning was just noise that fired on every /path computation outside
the populated set.

Now the `:not_found` clause silently returns the same fallback shape
without logging.

lib/microwaveprop_web/live/path_live.ex

@@ -347,17 +347,16 @@ defmodule MicrowavepropWeb.PathLive do
       # midpoint. Resolves thin trapping layers HRRR's 13 pressure levels
       # miss; feeds the 1.15× boost in Scorer.score_refractivity, gated on
       # Richardson so turbulent duct readings don't inflate the score.
+      #
+      # `:not_found` is the documented fallback when no native sample is
+      # within ±0.07° / ±1 h of the midpoint — the native profile table
+      # is sparsely populated (only points the per-QSO worker has
+      # touched). The empty-map fallback is correct behavior, not a
+      # failure, so we don't log it.
       native_duct =
         case Weather.nearest_native_duct_info(midlat, midlon, now) do
-          {:ok, info} ->
-            info
-
-          {:error, reason} ->
-            Logger.warning(
-              "PathLive nearest_native_duct_info failed: midpoint=#{midlat},#{midlon} reason=#{inspect(reason)}"
-            )
-
-            %{best_duct_band_ghz: nil, bulk_richardson: nil}
+          {:ok, info} -> info
+          {:error, :not_found} -> %{best_duct_band_ghz: nil, bulk_richardson: nil}
         end
 
       # Build conditions and score