fix(membership): add chronological sort key for custom :date fields

Custom :date values are real Date structs; sorting them by Erlang term
order compares day, then month, then year, so the member list ordered
them like day-first text instead of chronologically. Derive the sort key
from a single shared helper that maps a date to its Gregorian day count,
leaving the other value types at their already-correct natural order.

test/mv/membership/custom_field_sort_property_test.exs

@@ -0,0 +1,65 @@
+defmodule Mv.Membership.CustomFieldSortPropertyTest do
+  use ExUnit.Case, async: true
+  use ExUnitProperties
+
+  alias Mv.Membership.CustomFieldSort
+
+  defp date_generator do
+    gen all(
+          year <- integer(1..9999),
+          month <- integer(1..12),
+          day <- integer(1..28)
+        ) do
+      Date.new!(year, month, day)
+    end
+  end
+
+  # The production load path always delivers a :date value as
+  # %Ash.Union{type: :date, value: %Date{}}, so the property exercises both the
+  # bare %Date{} form and the union-wrapped form to pin the union-dispatch clause.
+  defp shape_generator do
+    member_of([:bare, :union])
+  end
+
+  defp wrap_date(date, :bare), do: date
+  defp wrap_date(date, :union), do: %Ash.Union{type: :date, value: date}
+
+  property "sort_key/2 orders :date values chronologically in both directions" do
+    check all(
+            raw_dates <- list_of(date_generator(), min_length: 1),
+            shape <- shape_generator()
+          ) do
+      values = Enum.map(raw_dates, &wrap_date(&1, shape))
+
+      ascending = Enum.sort_by(values, &CustomFieldSort.sort_key(&1, :date))
+      ascending_dates = Enum.map(ascending, &unwrap/1)
+      descending_dates = Enum.reverse(ascending_dates)
+
+      assert non_decreasing?(ascending_dates)
+      assert non_increasing?(descending_dates)
+
+      # The key must be an integer Gregorian-day count, not a stringified date:
+      # this pins the dedicated :date branch and guards against a string-coerced
+      # key whose chronological correctness would silently depend on zero-padded
+      # ISO formatting.
+      Enum.each(values, fn value ->
+        assert CustomFieldSort.sort_key(value, :date) == Date.to_gregorian_days(unwrap(value))
+      end)
+    end
+  end
+
+  defp unwrap(%Ash.Union{value: value}), do: value
+  defp unwrap(%Date{} = date), do: date
+
+  defp non_decreasing?(dates) do
+    dates
+    |> Enum.chunk_every(2, 1, :discard)
+    |> Enum.all?(fn [a, b] -> Date.compare(a, b) != :gt end)
+  end
+
+  defp non_increasing?(dates) do
+    dates
+    |> Enum.chunk_every(2, 1, :discard)
+    |> Enum.all?(fn [a, b] -> Date.compare(a, b) != :lt end)
+  end
+end

test/mv/membership/custom_field_sort_test.exs

@@ -0,0 +1,29 @@
+defmodule Mv.Membership.CustomFieldSortTest do
+  use ExUnit.Case, async: true
+
+  alias Mv.Membership.CustomFieldSort
+
+  describe "sort_key/2" do
+    test "keeps :integer values numerically comparable" do
+      values = [10, 100, 2]
+
+      sorted = Enum.sort_by(values, &CustomFieldSort.sort_key(&1, :integer))
+
+      assert sorted == [2, 10, 100]
+    end
+
+    test "passes :string values through to their natural term-order key" do
+      assert CustomFieldSort.sort_key("Zebra", :string) == "Zebra"
+    end
+
+    test "passes :email values through to their natural term-order key" do
+      assert CustomFieldSort.sort_key("a@example.com", :email) == "a@example.com"
+    end
+
+    test "unwraps an %Ash.Union{} value before deriving the key" do
+      union = %Ash.Union{type: :integer, value: 42}
+
+      assert CustomFieldSort.sort_key(union, :integer) == 42
+    end
+  end
+end