Add ProgressMuxer and DelegateProgress utilities

PowerKit.Tests/ProgressMuxerTests.cs

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+using FluentAssertions;
+using Gress;
+using PowerKit;
+using Xunit;
+
+namespace PowerKit.Tests;
+
+public class ProgressMuxerTests
+{
+    [Fact]
+    public void CreateInput_Test()
+    {
+        // Arrange
+        var progress = new ProgressCollector<double>();
+        var muxer = new ProgressMuxer(progress);
+        var input = muxer.CreateInput();
+
+        // Act
+        input.Report(0.5);
+        input.Report(1.0);
+
+        // Assert
+        progress.GetValues().Should().Equal(0.5, 1.0);
+    }
+
+    [Fact]
+    public void CreateInput_Weight_Test()
+    {
+        // Arrange
+        var progress = new ProgressCollector<double>();
+        var muxer = new ProgressMuxer(progress);
+        var input = muxer.CreateInput(weight: 0.5);
+
+        // Act
+        input.Report(0.5);
+        input.Report(1.0);
+
+        // Assert
+        progress.GetValues().Should().Equal(0.25, 0.5);
+    }
+
+    [Fact]
+    public void CreateInput_MultipleInputs_Test()
+    {
+        // Arrange
+        var progress = new ProgressCollector<double>();
+        var muxer = new ProgressMuxer(progress);
+        var input1 = muxer.CreateInput(weight: 0.6);
+        var input2 = muxer.CreateInput(weight: 0.4);
+
+        // Act
+        input1.Report(1.0);
+        input2.Report(1.0);
+
+        // Assert
+        var values = progress.GetValues();
+        values[^1].Should().BeApproximately(1.0, 1e-10);
+    }
+}

PowerKit/DelegateProgress.cs

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+#if NET40_OR_GREATER || NETSTANDARD || NET
+#nullable enable
+using System;
+using System.Diagnostics.CodeAnalysis;
+
+namespace PowerKit;
+
+/// <summary>
+/// Provides a lightweight <see cref="IProgress{T}" /> implementation that delegates
+/// progress reporting to an action.
+/// </summary>
+#if !POWERKIT_INCLUDE_COVERAGE
+[ExcludeFromCodeCoverage]
+#endif
+internal sealed class DelegateProgress<T>(Action<T> report) : IProgress<T>
+{
+    /// <inheritdoc />
+    public void Report(T value) => report(value);
+}
+#endif

PowerKit/ProgressMuxer.cs

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+#if NET40_OR_GREATER || NETSTANDARD || NET
+#nullable enable
+using System;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Linq;
+using System.Threading;
+
+namespace PowerKit;
+
+/// <summary>
+/// Multiplexes multiple <see cref="IProgress{T}" /> reporters into a single output reporter,
+/// combining weighted progress values from multiple sources.
+/// </summary>
+#if !POWERKIT_INCLUDE_COVERAGE
+[ExcludeFromCodeCoverage]
+#endif
+internal class ProgressMuxer(IProgress<double> output)
+{
+    private readonly Lock _lock = new();
+    private readonly Dictionary<int, double> _splitTotals = new();
+
+    private int _splitCount;
+
+    /// <summary>
+    /// Creates a new progress input with the specified weight.
+    /// Progress reported to this input is multiplied by <paramref name="weight" />
+    /// and combined with all other inputs before being forwarded to the output.
+    /// </summary>
+    public IProgress<double> CreateInput(double weight = 1)
+    {
+        var index = Interlocked.Increment(ref _splitCount) - 1;
+        return new DelegateProgress<double>(p =>
+        {
+            using (_lock.EnterScope())
+            {
+                _splitTotals[index] = weight * p;
+                output.Report(_splitTotals.Values.Sum());
+            }
+        });
+    }
+}
+#endif