feat: add FrameClock (virtual clock + per-frame op budget)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

engine/Age.Engine.Tests/FrameClockTests.cs

@@ -0,0 +1,29 @@
+using Age.Engine.Hosting;
+using Xunit;
+
+public class FrameClockTests
+{
+    [Fact]
+    public void Advance_AtSpeed1_AddsRealMilliseconds()
+    {
+        var c = new FrameClock();               // Speed defaults to 1.0
+        c.Advance(0.016);                        // one ~60fps frame
+        Assert.Equal(16, c.NowMs);
+    }
+
+    [Fact]
+    public void Advance_ScalesBySpeed()
+    {
+        var c = new FrameClock { Speed = 4.0 };
+        c.Advance(0.016);
+        Assert.Equal(64, c.NowMs);               // 4x virtual time
+    }
+
+    [Fact]
+    public void EffectiveBudget_ScalesBySpeed_AndFloorsAtOne()
+    {
+        Assert.Equal(30,  new FrameClock { OpsPerFrame = 30, Speed = 1.0 }.EffectiveBudget);
+        Assert.Equal(120, new FrameClock { OpsPerFrame = 30, Speed = 4.0 }.EffectiveBudget);
+        Assert.Equal(1,   new FrameClock { OpsPerFrame = 0,  Speed = 1.0 }.EffectiveBudget);
+    }
+}