Mash tool: Ctrl+C ends the run early with a full summary

Stopping early is a real operator workflow (finger fatigue); previously
an interrupt died summary-less and could orphan a process holding the
COM port. CancelKeyPress now breaks the run loop, snapshots the after-
counters, and prints the summary with the actual elapsed duration.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tools/RioSerialMonitor/MashTest.cs

@@ -224,6 +224,16 @@ internal static class MashTest
         using var cts = new CancellationTokenSource();
         Task run = link.RunAsync(cts.Token);
 
+        // Ctrl+C = "my fingers hurt": end the run NOW but still snapshot the
+        // counters and print the full summary, instead of dying summary-less
+        // (and leaving an orphan holding the port, as an interrupted run did).
+        bool stopRequested = false;
+        Console.CancelKeyPress += (_, e) =>
+        {
+            e.Cancel = true; // we shut down ourselves
+            stopRequested = true;
+        };
+
         // --- counters BEFORE ---------------------------------------------------
         await Task.Delay(500);
         Dictionary<RioStatusType, int> before = await SnapshotStatusAsync();
@@ -232,6 +242,7 @@ internal static class MashTest
         Raw("");
         Raw(">>> MASH NOW: two hands, 8 lamp buttons, as fast as you can. <<<");
         Raw($">>> Test runs {seconds}s. A wedge alarm will beep + banner.   <<<");
+        Raw(">>> Ctrl+C ends the run early and still prints the summary.   <<<");
         Raw("");
 
         // --- wedge watchdog + progress ticker ----------------------------------
@@ -239,6 +250,12 @@ internal static class MashTest
         TimeSpan nextProgress = sw.Elapsed + TimeSpan.FromSeconds(30);
         while (sw.Elapsed < runEnd)
         {
+            if (stopRequested)
+            {
+                seconds = (int)sw.Elapsed.TotalSeconds; // summary reflects reality
+                Log("operator stop (Ctrl+C) — ending run, snapshotting counters...");
+                break;
+            }
             await Task.Delay(100);
             lock (gate)
             {