feat(D.2b): UiMeter vital bar + fill-geometry tests

Adds UiMeter, the horizontal vital-bar widget for the D.2b retail-look
UI toolkit. Solid-color fill for Spec 1; the retail orb sprite + scissor
crop path is reserved for a later sub-phase. Five unit tests (1 Fact +
4 Theory) cover half-fill geometry and clamping at -1/0/1/2 fractions.

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

src/AcDream.App/UI/UiMeter.cs

@@ -0,0 +1,40 @@
+using System.Numerics;
+
+namespace AcDream.App.UI;
+
+/// <summary>
+/// A horizontal vital bar: an empty background rect with a partial-width
+/// fill. <see cref="Fill"/> returns 0..1 (or null = no data → empty bar).
+/// Solid-color for Spec 1; the retail orb sprite + scissor crop is a later
+/// sub-phase.
+/// </summary>
+public sealed class UiMeter : UiElement
+{
+    /// <summary>Fill fraction provider; a null result draws an empty bar.</summary>
+    public Func<float?> Fill { get; set; } = () => 0f;
+    public Vector4 BarColor { get; set; } = new(1f, 0f, 0f, 1f);
+    public Vector4 BgColor  { get; set; } = new(0f, 0f, 0f, 0.5f);
+
+    public UiMeter() { ClickThrough = true; }
+
+    /// <summary>Clamp <paramref name="pct"/> to [0,1] and return the fill rect
+    /// (local px) for a bar of <paramref name="w"/> x <paramref name="h"/>.</summary>
+    public static (float x, float y, float w, float h) ComputeFillRect(
+        float pct, float w, float h)
+    {
+        if (pct < 0f) pct = 0f;
+        if (pct > 1f) pct = 1f;
+        return (0f, 0f, w * pct, h);
+    }
+
+    protected override void OnDraw(UiRenderContext ctx)
+    {
+        ctx.DrawRect(0, 0, Width, Height, BgColor);
+        float? pct = Fill();
+        if (pct is float p)
+        {
+            var (fx, fy, fw, fh) = ComputeFillRect(p, Width, Height);
+            if (fw > 0f) ctx.DrawRect(fx, fy, fw, fh, BarColor);
+        }
+    }
+}