feat(D.2b): MarkupDocument (XML -> UiElement tree); vitals panel from vitals.xml

Implements Task 8 of the D.2b retail-UI plan. MarkupDocument.Build() parses
KSML-style panel markup into a live UiNineSlicePanel subtree, resolving
{Binding} attribute expressions against a supplied object via reflection.
Color format is #AARRGGBB (alpha-first, matching controls.ini). Handles
<panel> root (geometry + optional title label) and <meter> children (fill,
label, bar color). Future element kinds (label, button, image) extend the
switch without touching existing code.

vitals.xml encodes the just-approved vitals panel layout (health red #FFC70D0D,
stamina gold #FFD49E1F, mana blue #FF1F33D9); ships next to the binary via
PreserveNewest csproj rule. GameWindow.cs drops the 35-line hand-built panel
block in favour of a 4-line File.ReadAllText + MarkupDocument.Build call —
identical tree, identical render, now data-driven.

2 new tests (Build_CreatesPanelWithMeterFillLabelAndGeometry,
Build_NullBindingValuesYieldNullFillAndLabel) + 11 total targeted green.

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

src/AcDream.App/AcDream.App.csproj

@@ -50,6 +50,11 @@
     <None Include="..\..\docs\research\data\spells.csv" Link="data\spells.csv">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </None>
+    <!-- Phase D.2b: KSML-style panel markup assets (vitals.xml etc.) ship
+         next to the binary so MarkupDocument.Build can load them at runtime. -->
+    <None Include="UI\assets\**\*.xml">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </None>
   </ItemGroup>
   <ItemGroup>
     <!-- Build the smoke plugin first and copy it into plugins/AcDream.Plugins.Smoke/ -->