fix struct inconsistencies

lib/mv/membership/import/member_csv.ex

@@ -8,10 +8,10 @@ defmodule Mv.Membership.Import.MemberCSV do
 
   ## Error Handling
 
-  Errors are returned as `%Error{}` structs containing:
-  - `csv_line_number` - The physical line number in the CSV file
+  Errors are returned as `%MemberCSV.Error{}` structs containing:
+  - `csv_line_number` - The physical line number in the CSV file (or `nil` for general errors)
   - `field` - The field name (atom) or `nil` if not field-specific
-  - `message` - Human-readable error message
+  - `message` - Human-readable error message (or `nil` for general errors)
 
   ## Import State
 
@@ -26,7 +26,7 @@ defmodule Mv.Membership.Import.MemberCSV do
   The `chunk_result` returned by `process_chunk/3` contains:
   - `inserted` - Number of successfully created members
   - `failed` - Number of failed member creations
-  - `errors` - List of `%Error{}` structs (capped at 50 per import)
+  - `errors` - List of `%MemberCSV.Error{}` structs (capped at 50 per import)
 
   ## Examples
 
@@ -51,9 +51,9 @@ defmodule Mv.Membership.Import.MemberCSV do
     defstruct csv_line_number: nil, field: nil, message: nil
 
     @type t :: %__MODULE__{
-            csv_line_number: integer(),
+            csv_line_number: pos_integer() | nil,
             field: atom() | nil,
-            message: String.t()
+            message: String.t() | nil
           }
   end