basic arithmetic benchmarks added

2-benchmarks/Just.PreciseMath.Benchmarks/Program.cs

@@ -0,0 +1,31 @@
+using BenchmarkDotNet.Reports;
+using BenchmarkDotNet.Running;
+
+namespace Just.PreciseMath.Benchmarks;
+
+internal static class Program
+{
+    private static int Main(string[] args)
+    {
+        BenchmarkSwitcher switcher = BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly);
+        // Only these standalone discovery/help forms intentionally produce no reports.
+        if (args is ["--list", "flat" or "tree"] or ["--help"] or ["-h"])
+        {
+            switcher.Run(args);
+            return 0;
+        }
+
+        Summary[] summaries = switcher.Run(args.Length == 0 ? ["--filter", "*"] : args).ToArray();
+        // BenchmarkDotNet can return normally after validation, build, or execution
+        // failures. Do not let CI mistake an empty or failed run for a passing smoke test.
+        if (summaries.Length == 0 || summaries.Any(summary =>
+            summary.HasCriticalValidationErrors || summary.Reports.Length == 0 ||
+            summary.Reports.Any(report => !report.Success || report.ResultStatistics is null)))
+        {
+            Console.Error.WriteLine("Benchmark run failed or produced no measurements.");
+            return 1;
+        }
+
+        return 0;
+    }
+}