fix(pskr): drop spots with <6-char grids on either end

A 4-char locator covers ~70 km × 100 km, which dwarfs HRRR's 3 km
native cell and any midpoint we'd derive for the calibration corpus.
Storing those samples gives the recalibrator geometry that's
indistinguishable from noise.

fetch_grid/2 now returns {:error, {:short_grid, key, grid}} for any
locator under 6 chars after Maidenhead validation. The spot is
dropped before it reaches the aggregator, so neither pskr_spots_hourly
nor pskr_calibration_samples ever see it.

lib/microwaveprop/pskr.ex

@@ -152,16 +152,26 @@ defmodule Microwaveprop.Pskr do
     }
   end
 
-  # Keep locators at the precision the spotter reported. Maidenhead
-  # validity already enforces minimum 4 chars + even length + the
-  # right alphabet at each position, so anything that passes is a
-  # legal 4/6/8/10-char locator we can store as-is. Uppercase for
-  # consistent indexing — the unique key is case-sensitive.
+  # Require ≥ 6-char locators on both ends. 4-char grids cover
+  # ~70 km × 100 km, so the midpoint and path distance error
+  # dwarfs any HRRR cell match (3 km native) and the resulting
+  # calibration sample geometry is meaningless. Drop the spot
+  # entirely rather than store a low-precision row.
+  #
+  # Maidenhead validity enforces even length + correct alphabet
+  # per position, so a passing string is a legal 6/8/10-char
+  # locator we can store as-is. Uppercase for consistent indexing
+  # — the unique key is case-sensitive.
   defp fetch_grid(json, key) do
     case Map.get(json, key) do
       grid when is_binary(grid) ->
         upper = String.upcase(grid)
-        if Maidenhead.valid?(upper), do: {:ok, upper}, else: {:error, {:bad_grid, key, grid}}
+
+        cond do
+          not Maidenhead.valid?(upper) -> {:error, {:bad_grid, key, grid}}
+          String.length(upper) < 6 -> {:error, {:short_grid, key, grid}}
+          true -> {:ok, upper}
+        end
 
       other ->
         {:error, {:missing_grid, key, other}}

test/microwaveprop/pskr_test.exs

@@ -36,12 +36,16 @@ defmodule Microwaveprop.PskrTest do
       assert DateTime.to_unix(spot.transmit_time) == 1_662_407_697
     end
 
-    test "accepts 4-char grids verbatim" do
-      json = @sample |> Map.merge(%{"sl" => "EM12", "rl" => "DM43"}) |> Jason.encode!()
+    test "rejects 4-char grids on either end (insufficient precision)" do
+      # A 4-char locator is ~70 km × 100 km — the midpoint and
+      # distance error swamps any HRRR cell match (3 km native).
+      # Drop the spot rather than store a calibration sample whose
+      # geometry is meaningless.
+      sender_short = @sample |> Map.put("sl", "EM12") |> Jason.encode!()
+      receiver_short = @sample |> Map.put("rl", "DM43") |> Jason.encode!()
 
-      assert {:ok, spot} = Pskr.parse_spot(json)
-      assert spot.sender_grid == "EM12"
-      assert spot.receiver_grid == "DM43"
+      assert {:error, _} = Pskr.parse_spot(sender_short)
+      assert {:error, _} = Pskr.parse_spot(receiver_short)
     end
 
     test "preserves extended-square (8-char) precision" do