feat(D.5.3a): selected-object meter — Health bar + name on the action bar

Port of gmToolbarUI::HandleSelectionChanged (acclient_2013_pseudo_c.txt:198635).
When the player selects a world object the action bar's bottom strip shows the
object name + (for player/pet/attackable targets) a live Health meter; deselect
clears it. Mana (#140) + stack slider deferred.

- SelectedObjectController (new): clear-then-populate on selection change; sets
  name (UiText child, VitalsController pattern), overlay state (ObjectSelected /
  StackedItemSelected via UiDatElement.ActiveState), shows the health meter and
  sends QueryHealth for health targets. Subscribes via a delegate seam (no
  GameWindow coupling).
- GameWindow: _selectedGuid field -> SelectedGuid property + SelectionChanged
  event (fires on actual change only); 3 write sites converted, reads untouched.
  All selection-write paths (LMB pick, Tab/Q, despawn-clear via Tick()) run on
  the render thread, so the event-driven UI mutation is single-threaded.
- WorldSession.SendQueryHealth (0x01BF) — wraps SocialActions.BuildQueryHealth.
- DatWidgetFactory.BuildMeter: handle the single-image toolbar meter shape
  (back-track on the element's own DirectState, fill on one Type-3 child). The
  sprites go in the TILE slot (DrawMode=Normal tiles to full bar geometry per
  UIElement_Meter::DrawChildren) — a left-cap assignment would gap/clamp a
  sub-140px sprite. Vitals 3-slice path unchanged.
- ToolbarController.HiddenIds: A1 (health) now owned by SelectedObjectController;
  A2 (mana) + A4 (stack) stay hidden (deferred) so their dat back-tracks don't
  render as stray empty bars.

Adversarial Opus review found + fixed: the mana-meter orphan (A2 left unhidden)
and the meter tile-vs-cap render bug (C1). Divergence rows AP-46 (health gate
approximation: IsLiveCreatureTarget vs IsPlayer||pet||attackable) + AP-47
(meter shown on select vs on UpdateHealth reply). Spec §5 corrected.

Build + full test suite green (2,684 passed / 4 skipped). Health meter render
fidelity (full-width fill + fraction mapping) pending the user's visual gate.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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

@@ -136,6 +136,47 @@ public class DatWidgetFactoryTests
         Assert.IsType<AcDream.App.UI.UiItemList>(w);
     }
 
+    // ── Test M1: Single-image meter (toolbar selected-object meters) ────────
+    //
+    // The toolbar health/mana meters (0x100001A1 / 0x100001A2) use a DIFFERENT
+    // shape from the vitals 3-slice meters: the back-track sprite lives on the
+    // meter ELEMENT's own DirectState ("" key), and there is exactly ONE Type-3
+    // child whose own DirectState ("" key) carries the fill sprite.  That child
+    // has no image grandchildren, so SliceIds would return all-zero — the new
+    // Count==1 branch reads the StateMedia entries directly instead.
+    // The sprites go in the TILE slot (Back/FrontTile), NOT the cap slot: DrawMode=Normal
+    // tiles at native width across the full bar geometry (UIElement_Meter::DrawChildren),
+    // so the back spans all 140px and the fill clips to 140*fraction for any native width.
+    // Back/FrontLeft + Back/FrontRight must be 0 (no caps on a single-image bar).
+
+    [Fact]
+    public void BuildMeter_SingleImageShape_ReadsDirectStateFromElementAndFillChild()
+    {
+        const uint BackFile = 0x0600193Eu;  // health back-track (from toolbar dump)
+        const uint FillFile = 0x0600193Fu;  // health fill (from toolbar dump)
+
+        // Meter element: Type 7, own DirectState = back-track sprite.
+        var meter = new ElementInfo { Type = 7, Id = 0x100001A1u, Width = 140, Height = 31 };
+        meter.StateMedia[""] = (BackFile, 1);
+
+        // Single Type-3 fill container: own DirectState = fill sprite, no grandchildren.
+        var fillContainer = new ElementInfo { Type = 3, ReadOrder = 1 };
+        fillContainer.StateMedia[""] = (FillFile, 1);
+        meter.Children.Add(fillContainer);
+
+        var e = DatWidgetFactory.Create(meter, NoTex, null);
+
+        var m = Assert.IsType<UiMeter>(e);
+        // Back-track on the meter element's own DirectState, fill on the single child —
+        // both in the TILE slot so they tile across the full 140px bar (DrawMode=Normal).
+        Assert.Equal(BackFile, m.BackTile);
+        Assert.Equal(0u,       m.BackLeft);
+        Assert.Equal(0u,       m.BackRight);
+        Assert.Equal(FillFile, m.FrontTile);
+        Assert.Equal(0u,       m.FrontLeft);
+        Assert.Equal(0u,       m.FrontRight);
+    }
+
     // ── Test 6: Meter slice extraction (the important one) ───────────────────
 
     /// <summary>