refactor(kpi): K4/K10/K12 review fixups — test data-race + faulted-tick liveness, dead-branch/unused removal, NaN-guard assertions, value clamp + doc

tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Components/Shared/KpiTrendChartTests.cs

@@ -106,7 +106,12 @@ public class KpiTrendChartTests : BunitContext
             .Add(c => c.Title, "Flat Time")
             .Add(c => c.IsAvailable, true));
 
-        Assert.Contains("<polyline", cut.Markup);
+        var markup = cut.Markup;
+        Assert.Contains("<polyline", markup);
+        // The even-spacing fallback must produce finite coordinates — no NaN or Infinity
+        // from the divide-by-zero guard path.
+        Assert.DoesNotContain("NaN", markup);
+        Assert.DoesNotContain("Infinity", markup);
     }
 
     [Fact]
@@ -123,7 +128,12 @@ public class KpiTrendChartTests : BunitContext
             .Add(c => c.Title, "Quiet")
             .Add(c => c.IsAvailable, true));
 
-        Assert.Contains("<polyline", cut.Markup);
+        var markup = cut.Markup;
+        Assert.Contains("<polyline", markup);
+        // The zero-max flat-line guard must produce finite coordinates — no NaN or Infinity
+        // from the yFrac divide-by-zero protection path.
+        Assert.DoesNotContain("NaN", markup);
+        Assert.DoesNotContain("Infinity", markup);
     }
 
     [Theory]