Add Instrumentation utility class for test output tracing

Introduces Trace(), TraceTiming(), and TraceCounter() static methods
for emitting [INSTRUMENT] lines visible in test output. Includes
InstrumentationTests exercising all three methods.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Laurian Avrigeanu

InkkSlinger.Tests/Instrumentation.cs

@@ -0,0 +1,38 @@
+using System.Diagnostics;
+
+namespace InkkSlinger.Tests;
+
+public static class Instrumentation
+{
+    /// <summary>
+    /// Writes a line to the test output that can be captured by InstrumentationCapture
+    /// and also appears in the test console output.
+    /// </summary>
+    [DebuggerStepThrough]
+    public static void Trace(string message)
+    {
+        var line = $"[INSTRUMENT] {message}";
+        System.Diagnostics.Trace.WriteLine(line);
+        Console.Out.WriteLine(line);
+    }
+
+    /// <summary>
+    /// Writes a timing entry in the standard format parsed by InstrumentationCapture.TryParseTiming.
+    /// Example: "MyMethod took 1234us"
+    /// </summary>
+    [DebuggerStepThrough]
+    public static void TraceTiming(string method, long microseconds)
+    {
+        Trace($"{method} took {microseconds}us");
+    }
+
+    /// <summary>
+    /// Writes a counter entry in the standard format parsed by InstrumentationCapture.TryParseCounter.
+    /// Example: "MyMethod #42"
+    /// </summary>
+    [DebuggerStepThrough]
+    public static void TraceCounter(string method, int count)
+    {
+        Trace($"{method} #{count}");
+    }
+}

InkkSlinger.Tests/InstrumentationTests.cs

@@ -0,0 +1,32 @@
+using Xunit;
+
+namespace InkkSlinger.Tests;
+
+public sealed class InstrumentationTests
+{
+    [Fact]
+    public void Trace_WritesLineToConsole()
+    {
+        Instrumentation.Trace("Test message from InstrumentationTests");
+    }
+
+    [Fact]
+    public void TraceTiming_WritesTimingLine()
+    {
+        Instrumentation.TraceTiming("FakeMethod", 12345);
+    }
+
+    [Fact]
+    public void TraceCounter_WritesCounterLine()
+    {
+        Instrumentation.TraceCounter("FakeMethod", 42);
+    }
+
+    [Fact]
+    public void Trace_MultipleLines_AreOrdered()
+    {
+        Instrumentation.Trace("Line 1");
+        Instrumentation.Trace("Line 2");
+        Instrumentation.Trace("Line 3");
+    }
+}