feat(D.2b): per-window resize-axis lock; vitals window is X-only (retail)

Add ResizeX/ResizeY bool properties to UiElement (both true by default).
HitEdges() in UiRoot masks out locked axes after edge detection, so a
locked edge falls through to window-move behaviour — matching retail,
where the vitals bar height is fixed and only widens.

MarkupDocument.Build() parses an optional resize="x|y|both|none"
attribute on <panel>; vitals.xml gets resize="x" to enforce the
horizontal-only constraint in all instances of the panel.

Two new tests: HitEdges_RespectsResizeAxisLock (UiRootInputTests) and
Build_ResizeAttrX_SetsHorizontalOnly (MarkupDocumentTests). 11/11 green.

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

tests/AcDream.App.Tests/UI/UiRootInputTests.cs

@@ -102,4 +102,14 @@ public class UiRootInputTests
         Assert.Equal(120f, panel.Top);
         root.OnMouseUp(UiMouseButton.Left, 220, 170);
     }
+
+    [Fact]
+    public void HitEdges_RespectsResizeAxisLock()
+    {
+        var panel = new UiPanel { Left = 100, Top = 100, Width = 200, Height = 100, ResizeY = false };
+        // right edge still detected (X allowed)
+        Assert.True((UiRoot.HitEdges(panel, 300, 150, 5) & ResizeEdges.Right) != 0);
+        // bottom edge masked out (Y locked)
+        Assert.True((UiRoot.HitEdges(panel, 200, 200, 5) & ResizeEdges.Bottom) == 0);
+    }
 }