fix: CycleGenerator generates from last cycle, not filling gaps

- Change algorithm to start from last existing cycle instead of start_date
- Deleted cycles (gaps) are no longer automatically filled
- Add test to verify gaps remain unfilled
- Update documentation to clarify gap handling behavior

test/mv/membership_fees/cycle_generator_test.exs

@@ -174,6 +174,63 @@ defmodule Mv.MembershipFees.CycleGeneratorTest do
       assert second_cycles == []
     end
 
+    test "does not fill gaps when cycles were deleted" do
+      setup_settings(true)
+      fee_type = create_fee_type(%{interval: :yearly})
+
+      # Create member without fee type first to control which cycles exist
+      member =
+        create_member_without_cycles(%{
+          join_date: ~D[2020-03-15],
+          membership_fee_start_date: ~D[2020-01-01]
+        })
+
+      # Manually create cycles for 2020, 2021, 2022, 2023
+      for year <- [2020, 2021, 2022, 2023] do
+        MembershipFeeCycle
+        |> Ash.Changeset.for_create(:create, %{
+          cycle_start: Date.new!(year, 1, 1),
+          member_id: member.id,
+          membership_fee_type_id: fee_type.id,
+          amount: fee_type.amount,
+          status: :unpaid
+        })
+        |> Ash.create!()
+      end
+
+      # Delete the 2021 cycle (create a gap)
+      cycle_2021 =
+        MembershipFeeCycle
+        |> Ash.Query.filter(member_id == ^member.id and cycle_start == ^~D[2021-01-01])
+        |> Ash.read_one!()
+
+      Ash.destroy!(cycle_2021)
+
+      # Now assign fee type to member (this triggers generation)
+      # Since cycles already exist (2020, 2022, 2023), the generator will
+      # start from the last existing cycle (2023) and go forward
+      member =
+        member
+        |> Ash.Changeset.for_update(:update_member, %{membership_fee_type_id: fee_type.id})
+        |> Ash.update!()
+
+      # Verify gap was NOT filled and new cycles were generated from last existing
+      all_cycles = get_member_cycles(member.id)
+      all_cycle_years = Enum.map(all_cycles, & &1.cycle_start.year) |> Enum.sort()
+
+      # 2021 should NOT exist (gap was not filled)
+      refute 2021 in all_cycle_years, "Gap at 2021 should NOT be filled"
+
+      # 2020, 2022, 2023 should exist (original cycles)
+      assert 2020 in all_cycle_years
+      assert 2022 in all_cycle_years
+      assert 2023 in all_cycle_years
+
+      # 2024 and 2025 should exist (generated after last existing cycle 2023)
+      assert 2024 in all_cycle_years
+      assert 2025 in all_cycle_years
+    end
+
     test "sets correct amount from membership fee type" do
       setup_settings(true)
       amount = Decimal.new("75.50")