Fix flashing borders not clearing after unit placement in Shardok

Two issues fixed:

1. PerformPlacementAction() was missing overlay cleanup after placing a unit.
   Added hexGrid.ClearOverlays() and HandleEnemyStartingPositionOverlays().

2. SelectedReserveUnitChangedTo() didn't clear board selection when switching
   from a board unit to a reserve unit. Now clears _selectedGridIndex and
   overlays before drawing new placement options.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/main/csharp/net/eagle0/clients/unity/eagle0/Assets/Shardok/ShardokGameController.cs

@@ -2157,6 +2157,10 @@ namespace Shardok {
                 throw new ArgumentException("either reserve unit or start index must be set");
 
             Model.PerformPlaceOrHideCommand(unit, GridIndexToMapCoords(finishGridIndex));
+
+            // Clear flashing borders after placement (same cleanup as PerformAction)
+            hexGrid.ClearOverlays();
+            HandleEnemyStartingPositionOverlays();
         }
 
         public void HandleHover(int gridIndex) {
@@ -2202,6 +2206,10 @@ namespace Shardok {
 
         public void SelectedReserveUnitChangedTo(UnitViewWithName unit) {
             if (unit != null) {
+                // Clear any board unit selection when selecting a reserve unit
+                _selectedGridIndex = null;
+                hexGrid.ClearOverlays();
+
                 var cmdGroup = commandTypeUIManager.CommandGroupForType(
                         Model.InSetUp ? CommandType.PlaceUnitCommand
                                       : CommandType.ReinforceCommand);
@@ -2209,6 +2217,8 @@ namespace Shardok {
                 RedrawCommandOverlays(null, null);
             } else {
                 SetDisplayedCommandGroup(0);
+                hexGrid.ClearOverlays();
+                HandleEnemyStartingPositionOverlays();
             }
         }