refactor

lib/mv/membership/import/header_mapper.ex

@@ -124,6 +124,45 @@ defmodule Mv.Membership.Import.HeaderMapper do
     end)
   end
 
+  @doc """
+  Returns a MapSet of normalized member field names.
+
+  This is the single source of truth for known member fields.
+  Used to distinguish between member fields and custom fields.
+
+  ## Returns
+
+  - `MapSet.t(String.t())` - Set of normalized member field names
+
+  ## Examples
+
+      iex> HeaderMapper.known_member_fields()
+      #MapSet<["email", "firstname", "lastname", "street", "postalcode", "city"]>
+  """
+  @spec known_member_fields() :: MapSet.t(String.t())
+  def known_member_fields do
+    cached = Process.get({__MODULE__, :known_member_fields})
+
+    if cached do
+      cached
+    else
+      fields =
+        @member_field_variants_raw
+        |> Map.keys()
+        |> Enum.map(fn canonical ->
+          # Normalize the canonical field name (e.g., :first_name -> "firstname")
+          canonical
+          |> Atom.to_string()
+          |> String.replace("_", "")
+          |> String.downcase()
+        end)
+        |> MapSet.new()
+
+      Process.put({__MODULE__, :known_member_fields}, fields)
+      fields
+    end
+  end
+
   @doc """
   Normalizes a CSV header string for comparison.