Reorder the tree rows when the assembly list is sorted

[siegfriedpammer]

Sorting the assembly list reorders it in place and reports each step as a
Move. Nothing downstream could act on that: AssemblyListTreeNode's handler
had cases for Add, Remove and Reset only, and neither
SharpTreeNodeCollection nor TreeFlattener had a move at all, so the rows kept
their pre-sort order until something else forced a rebuild.

This adds the move to all three layers:

• SharpTreeNodeCollection.Move(oldIndex, newIndex) reorders the backing list
  and raises a single NotifyCollectionChangedAction.Move.
• SharpTreeNode.OnChildrenChanged handles it without the attach/detach work the
  other actions need - the node keeps its parent and its visibility - and moves
  the node's run in the flat list in one step.
• TreeFlattener.NodesMoved raises one ranged Move towards the ItemsSource.
• AssemblyListTreeNode reorders its children instead of ignoring the
  notification.

Moving the node rather than removing and re-inserting it keeps its identity, so
an expanded subtree stays expanded and its row is not rebuilt.

The one thing worth a second opinion

A run longer than one row has to be reported the way the consumer reads it:
Avalonia's VirtualizingStackPanel applies a ranged move by removing
OldItems.Count rows and re-inserting them at NewStartingIndex - (Count - 1).
A run reported by its final start index therefore lands one run-length short -
the headless test in this PR renders A, C, B, C instead of A, C, B, B1 if
NodesMoved reports the start. It now reports the row the run ends on for a
forward move, which is documented at the call site. For a single row - a
collapsed node, and every move a sort makes - the two readings coincide.

Also worth knowing: a Move does not carry the selection.
SelectionNodeBase.OnSourceCollectionChanged in Avalonia handles Move as
remove-then-add and drops the selection of the moved items. Restoring it would be
a TreeSelectionBinder change and is not part of this PR.

Tests

Five model tests in FlatListTreeNodeTests (event shape, no-op move, flattened
order, expanded-run move, ranged-event indices) and one headless rendering test in
SharpTreeViewTests. Full ILSpy.Tests run: 1283 tests, 0 failed, 3 skipped.

Opened by an AI agent (Claude) on Siegfried's behalf.