fix(ui): world tooltips never cleared, stacking dozens of popups — single-slot invariant restored on every found-object edge

RetailTooltipPresenter.UpdateWorldHoverTooltip only called RemovePopup()
on the found-object-LOST edge (found == 0u). An A->B found-object CHANGE
(walking past a run of NPCs/doors/lifestones with no intervening "nothing
found" frame) skipped straight to TryBuildAndMountPopup with the previous
popup still mounted as a child of _host -- only the _popupRoot reference
got overwritten, so every earlier popup was orphaned in the tree and never
removed. Matches the user's screenshot of 15+ stacked name boxes.

Fix: clear any showing world popup on ANY found-object edge -- change or
loss -- before evaluating whether to mount a new one, mirroring
OnTooltipShow's own unconditional RemovePopup() at its top.

Live-verified against local ACE (testaccount/+Acdream, session-config
launch): a temporary probe logged 103 mount/102 remove events across many
direct object-to-object transitions (Silver Tusker, Armored Tusker,
+Acdream); hostChildren never exceeded baseline+1 and popupSkinChildren
never exceeded 1 -- confirmed at most one tooltip ever exists. Probe
stripped before landing; two new fixture regressions
(WorldHover_FoundObjectChangesDirectly_ReplacesThePopupWithoutStacking,
WorldHover_ThenUiDwellTooltip_ReplacesRatherThanStacks) both fail pre-fix.

fix #409 (follow-on)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/AcDream.App.Tests/UI/Layout/RetailTooltipPresenterTests.cs

@@ -689,6 +689,76 @@ public sealed class RetailTooltipPresenterTests
         Assert.Empty(requests);
     }
 
+    [Fact]
+    public void WorldHover_FoundObjectChangesDirectly_ReplacesThePopupWithoutStacking()
+    {
+        // #409 follow-on (2026-08-16 overnight hover/UI round, Batch A bug 1):
+        // the regression that filled the user's screen with dozens of
+        // stacked tooltips. Walking past a run of NPCs/doors/lifestones never
+        // produces a frame where the found guid is 0 — it goes straight from
+        // A to B to C. RecvNotice_SmartBoxObjectFound-equivalent must still
+        // only ever have ONE popup mounted: found A, then found B (no
+        // intervening "nothing found" tick) must swap the popup, not add a
+        // second one on top of the first.
+        const uint otherGuid = 0x80000456u;
+        var (root, presenter, requests) = CreateHarness();
+        uint current = WorldFoundGuid;
+        presenter.WorldHoverGuidProvider = () => current;
+        presenter.WorldHoverNameResolver = guid =>
+            guid == WorldFoundGuid ? "A Drudge" : "A Door";
+        presenter.WorldTooltipsEnabled = () => true;
+        int childrenBefore = root.Children.Count;
+
+        presenter.Tick();
+        Assert.Equal(childrenBefore + 1, root.Children.Count);
+
+        current = otherGuid;
+        presenter.Tick();
+
+        // Exactly one popup, not two stacked.
+        Assert.Equal(childrenBefore + 1, root.Children.Count);
+        Assert.Equal(2, requests.Count);
+
+        current = WorldFoundGuid;
+        presenter.Tick();
+        current = otherGuid;
+        presenter.Tick();
+        current = WorldFoundGuid;
+        presenter.Tick();
+
+        // Several more A/B/A swaps still leave exactly one popup mounted —
+        // this is the "dozens of stacked name boxes" scenario, minus the bug.
+        Assert.Equal(childrenBefore + 1, root.Children.Count);
+    }
+
+    [Fact]
+    public void WorldHover_ThenUiDwellTooltip_ReplacesRatherThanStacks()
+    {
+        // The other half of the "no stacking" contract: a world tooltip
+        // showing, then the mouse settles on a real UI element (dwell path)
+        // — OnTooltipShow's own unconditional RemovePopup() must clear the
+        // world popup, leaving exactly one popup (the UI one), not two.
+        var (root, presenter, _) = CreateHarness();
+        presenter.WorldHoverGuidProvider = () => WorldFoundGuid;
+        presenter.WorldHoverNameResolver = _ => "A Drudge";
+        presenter.WorldTooltipsEnabled = () => true;
+        int childrenBefore = root.Children.Count;
+
+        presenter.Tick();
+        Assert.Equal(childrenBefore + 1, root.Children.Count); // world tooltip up
+
+        var target = AddFullyAuthoredTarget(root);
+        root.OnMouseMove(110, 110);
+        root.Tick(0.016, 0);
+        root.Tick(0.016, root.TooltipDelayMs);
+
+        UiElement popup = root.Children.Single(c => !ReferenceEquals(c, target));
+        Assert.NotNull(popup);
+        // childrenBefore world-target(none) + target(1) + popup(1) == +2 total,
+        // never +3 (world popup replaced, not stacked).
+        Assert.Equal(childrenBefore + 2, root.Children.Count);
+    }
+
     [Fact]
     public void WorldHover_ReEvaluatesGateAndTextOnlyOnTheFoundGuidEdge()
     {