Correct restore of order of items in target collections

[Brunni]

The current implementation of Automapper.Collection does not re-order the items in the target based on the items in the source collection. The order is ignored completely. This might be ok for Collections but not for Lists.

Furthermore, the support for ObservableCollection should be added, which allows to provide only minimal events for list order changes.

About my use case
We are using this Automapper to map DTOs to a WPF ViewModel (with Fody Propertychanged notifications) using live updates. The model includes a large list of items, which shifts itself, when items are inserted or appended. The WPF should only repaint those items which are actually changed and keep the UI as stable as possible.

What changed

• High-level algorithm:
  1. Build a lookup of current destination items keyed by equivalentComparer.GetHashCode(x) and collect candidates.
  2. Iterate source in order. For each source item, find a matching destination instance (via IsEquivalent). If found, update it in-place and push it into an ordered list; if not found, map a new destination item and push that into ordered and toAdd.
  3. Remove any remaining (unmatched) destination items.
  4. Ensure all newly created items are added to the destination.
  5. Reorder the destination to match the ordered sequence:
     • If destination is IList<T>, perform in-place reordering:
       • For ObservableCollection<T>, use Move(oldIndex, newIndex) to generate a single Move event per item.
       • For other IList<T>, remove/insert to move or insert missing items at the correct position.
     • For non-IList<T> fall back to remove-all then re-add in the desired order.

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