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

tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Shared/InstancePickerTests.cs

@@ -355,4 +355,69 @@ public class InstancePickerTests : BunitContext
         Assert.Equal(new[] { "Line1", "Zulu" }, Rows(cut));
         Assert.Empty(cut.FindAll("[data-instance-id='7']"));
     }
+
+    [Fact]
+    public void Toggle_WhileOpen_ClosesThePanel()
+    {
+        var cut = RenderPicker();
+
+        Open(cut);
+        Assert.Single(cut.FindAll("[data-test='t-panel']"));
+
+        Open(cut);
+
+        Assert.Empty(cut.FindAll("[data-test='t-panel']"));
+        Assert.Equal("false", cut.Find("[data-test='t-input']").GetAttribute("aria-expanded"));
+    }
+
+    [Fact]
+    public void TheFieldSitsAboveTheBackdrop_SoItStaysClickableWhileOpen()
+    {
+        // The backdrop is a fixed full-viewport layer; the stylesheet lifts the field above
+        // it through this class. Without it a click on the input or the chevron would land on
+        // the backdrop instead and close the panel. Only the class can be pinned here — the
+        // z-index itself is scoped CSS, verified visually.
+        var cut = RenderPicker();
+        Open(cut);
+
+        var field = cut.Find("[data-test='t-picker'] .input-group");
+        Assert.Contains("ip-field", field.ClassList);
+        Assert.Single(cut.FindAll("[data-test='t-backdrop']"));
+    }
+
+    [Fact]
+    public void TwoPickers_GetDistinctPanelIds()
+    {
+        // The panel id (and the field's aria-controls pointing at it) is minted per component
+        // instance, so two pickers on one page cannot collide on a DOM id.
+        var first = RenderPicker();
+        var second = RenderPicker();
+
+        Open(first);
+        Open(second);
+
+        var firstPanel = first.Find("[data-test='t-panel']").Id;
+        var secondPanel = second.Find("[data-test='t-panel']").Id;
+
+        Assert.False(string.IsNullOrEmpty(firstPanel));
+        Assert.NotEqual(firstPanel, secondPanel);
+        Assert.Equal(firstPanel, first.Find("[data-test='t-input']").GetAttribute("aria-controls"));
+        Assert.Equal(secondPanel, second.Find("[data-test='t-input']").GetAttribute("aria-controls"));
+    }
+
+    [Fact]
+    public void SameInstancesReference_LeavesTheTypedTextAndOpenPanelAlone()
+    {
+        // Complement of SiteChange_NewInstancesList_ClearsAStaleSelectionText: an unrelated
+        // parameter change is NOT a site change, so a half-typed filter must survive it.
+        var cut = RenderPicker();
+        Open(cut);
+        Type(cut, "zu");
+
+        cut.Render(ps => ps.Add(p => p.Placeholder, "Pick an instance"));
+
+        Assert.Equal("zu", InputValue(cut));
+        Assert.Single(cut.FindAll("[data-test='t-panel']"));
+        Assert.Equal(new[] { "Zulu" }, Rows(cut));
+    }
 }