separate the tracking parts of TL into a reusable base, so that TL becomes a 'renderer'

src/Build/Logging/TerminalLogger/StopwatchAbstraction.cs

@@ -3,7 +3,7 @@
 
 namespace Microsoft.Build.Logging;
 
-internal abstract class StopwatchAbstraction
+public abstract class StopwatchAbstraction
 {
     public abstract void Start();
     public abstract void Stop();

src/Build/Logging/TerminalLogger/TerminalBuildData.cs

@@ -0,0 +1,51 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+
+namespace Microsoft.Build.Logging;
+
+/// <summary>
+/// Tracks build-level data for the TerminalLogger across an entire build session.
+/// </summary>
+public sealed class TerminalBuildData
+{
+    /// <summary>
+    /// The timestamp of the build start event.
+    /// </summary>
+    public DateTime BuildStartTime { get; set; }
+
+    /// <summary>
+    /// Number of build errors encountered during the build.
+    /// </summary>
+    public int BuildErrorsCount { get; set; }
+
+    /// <summary>
+    /// Number of build warnings encountered during the build.
+    /// </summary>
+    public int BuildWarningsCount { get; set; }
+
+    /// <summary>
+    /// The project build context corresponding to the Restore initial target, or null if the build is currently not restoring.
+    /// </summary>
+    public int? RestoreContext { get; set; }
+
+    /// <summary>
+    /// True if restore failed and this failure has already been reported.
+    /// </summary>
+    public bool RestoreFailed { get; set; }
+
+    /// <summary>
+    /// True if restore happened and finished.
+    /// </summary>
+    public bool RestoreFinished { get; set; }
+
+    /// <summary>
+    /// Initializes a new instance of TerminalBuildData.
+    /// </summary>
+    /// <param name="buildStartTime">The timestamp when the build started.</param>
+    public TerminalBuildData(DateTime buildStartTime)
+    {
+        BuildStartTime = buildStartTime;
+    }
+}

src/Build/Logging/TerminalLogger/TerminalBuildMessage.cs

@@ -6,5 +6,5 @@ namespace Microsoft.Build.Logging;
 /// <summary>
 /// Represents a piece of diagnostic output (message/warning/error).
 /// </summary>
-internal record struct TerminalBuildMessage(TerminalMessageSeverity Severity, string Message)
+public record struct TerminalBuildMessage(TerminalMessageSeverity Severity, string Message)
 { }