fix(central-ui): InstancePicker — field above the backdrop; site change always closes; reference contract documented; tests

Three Task 2 review findings:

1. The outside-click backdrop is a fixed full-viewport layer at z-index 1040,
   but the input-group was unpositioned — so once the panel was open a click on
   the field or the chevron hit the backdrop and closed the panel instead of
   letting the operator keep typing. The field now carries .ip-field
   (position:relative, z-index 1042), stacking backdrop < panel < field. Pinned
   in bUnit by the class (the z-index itself is scoped CSS, verified visually).

2. OnParametersSet closes the panel on ANY lists change, not only when the
   selection disappeared: an open panel is showing the previous site's tree,
   which is wrong even when the selected instance happens to exist in both.
   The reference-equality contract on Areas/Instances is now stated on both
   parameters — pass a stable reference, replace it only when the set changed.

3. Tests: the chevron closes an open panel; two pickers mint distinct panel ids
   (and matching aria-controls); an unrelated parameter change with the SAME
   Instances reference leaves a half-typed filter and the open panel alone.

Claude-Session: https://claude.ai/code/session_01NcMG6xXdsJbmD2wYAgkK29

src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Shared/InstancePicker.razor

@@ -13,7 +13,8 @@
 @using ZB.MOM.WW.ScadaBridge.Commons.Entities.Instances
 
 <div class="ip-wrap position-relative" data-test="@($"{DataTestPrefix}-picker")">
-    <div class="input-group input-group-sm">
+    @* ip-field lifts the field above the open panel's backdrop — see the .razor.css note. *@
+    <div class="input-group input-group-sm ip-field">
         <input type="text" class="form-control" role="combobox" autocomplete="off"
                id="@_inputId" aria-autocomplete="list" aria-haspopup="tree"
                aria-expanded="@(_open ? "true" : "false")" aria-controls="@_panelId"