Fix unlink-by-omission: on_missing :ignore, test, doc, string-key

- Member update_member: on_missing :unrelate → :ignore (no unlink when :user omitted)
- Test: normal_user update linked member without :user keeps link
- Doc: unlink only explicit (user: nil), admin-only; Actor.admin?(nil) note
- Check: defense-in-depth for "user" string key

test/mv/membership/member_policies_test.exs

@@ -505,6 +505,35 @@ defmodule Mv.Membership.MemberPoliciesTest do
                Membership.update_member(linked_member, %{user: nil}, actor: normal_user)
     end
 
+    test "normal_user update linked member without :user keeps link", %{
+      normal_user: normal_user,
+      admin: admin,
+      unlinked_member: unlinked_member
+    } do
+      # Admin links member to a user
+      link_target =
+        Mv.Fixtures.user_with_role_fixture("own_data")
+        |> Mv.Authorization.Actor.ensure_loaded()
+
+      {:ok, linked_member} =
+        Membership.update_member(
+          unlinked_member,
+          %{user: %{id: link_target.id}},
+          actor: admin
+        )
+
+      # normal_user updates only first_name (no :user) – link must remain (on_missing: :ignore)
+      {:ok, updated} =
+        Membership.update_member(linked_member, %{first_name: "Updated"}, actor: normal_user)
+
+      assert updated.first_name == "Updated"
+
+      {:ok, user} =
+        Ash.get(Mv.Accounts.User, link_target.id, domain: Mv.Accounts, actor: admin)
+
+      assert user.member_id == updated.id
+    end
+
     test "admin can create member with :user argument", %{admin: admin} do
       link_target =
         Mv.Fixtures.user_with_role_fixture("own_data")