Add MemoryBackedStream

PowerKit.Tests/MemoryReadStreamTests.cs

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+using System.IO;
+using FluentAssertions;
+using Xunit;
+
+namespace PowerKit.Tests;
+
+public class MemoryReadStreamTests
+{
+    [Fact]
+    public void MemoryReadStream_MakesUnseekableStreamSeekable_Test()
+    {
+        // Arrange — write known data and open as a non-seekable network-style stream
+        var data = new byte[] { 1, 2, 3, 4, 5 };
+        using var tempFile = TempFile.Create();
+        File.WriteAllBytes(tempFile.Path, data);
+        using var source = File.OpenRead(tempFile.Path);
+
+        // Act
+        using var result = new MemoryReadStream(source);
+
+        // Assert — wrapper is always seekable and can re-read from the beginning
+        result.CanSeek.Should().BeTrue();
+
+        var partial = new byte[2];
+        result.ReadExactly(partial);
+
+        result.Seek(0, SeekOrigin.Begin);
+
+        var full = new byte[data.Length];
+        result.ReadExactly(full);
+        full.Should().Equal(data);
+    }
+}

PowerKit.Tests/MemoryWriteStreamTests.cs

@@ -0,0 +1,33 @@
+using System.IO;
+using FluentAssertions;
+using Xunit;
+
+namespace PowerKit.Tests;
+
+public class MemoryWriteStreamTests
+{
+    [Fact]
+    public void MemoryWriteStream_MakesUnseekableStreamSeekable_Test()
+    {
+        // Arrange — write-only file stream is not seekable after the initial position
+        var data = new byte[] { 1, 2, 3, 4, 5 };
+        using var tempFile = TempFile.Create();
+
+        // Act — wrapper buffers writes in memory; Flush() commits them to the source
+        using (var source = File.OpenWrite(tempFile.Path))
+        using (var wrapper = new MemoryWriteStream(source))
+        {
+            wrapper.CanSeek.Should().BeTrue();
+
+            // Write all bytes then seek back and overwrite the first two
+            wrapper.Write(data, 0, data.Length);
+            wrapper.Seek(0, SeekOrigin.Begin);
+            wrapper.Write(new byte[] { 10, 20 }, 0, 2);
+
+            wrapper.Flush();
+        }
+
+        // Assert — overwritten prefix is reflected in the file
+        File.ReadAllBytes(tempFile.Path).Should().Equal(new byte[] { 10, 20, 3, 4, 5 });
+    }
+}

PowerKit/MemoryReadStream.cs

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+using System.IO;
+
+namespace PowerKit;
+
+/// <summary>
+/// A <see cref="Stream" /> wrapper that lazily loads a readable stream into an in-memory buffer,
+/// making it fully seekable regardless of whether the underlying stream supports seeking.
+/// </summary>
+/// <remarks>
+/// <para>
+/// The underlying stream is read into memory on the first access to
+/// <see cref="Read" />, <see cref="Seek" />, <see cref="Length" />, or <see cref="Position" />.
+/// Subsequent operations work directly against the in-memory buffer.
+/// </para>
+/// <para>
+/// Content is buffered from the source stream's current position when the buffer is first
+/// initialized. The buffer position always starts at 0.
+/// </para>
+/// </remarks>
+public sealed class MemoryReadStream(Stream source) : Stream
+{
+    private MemoryStream? _buffer;
+
+    private MemoryStream EnsureBuffer()
+    {
+        if (_buffer is not null)
+            return _buffer;
+
+        var capacity = source.CanSeek ? (int)(source.Length - source.Position) : 0;
+        _buffer = new MemoryStream(capacity);
+        source.CopyTo(_buffer);
+        _buffer.Position = 0;
+
+        return _buffer;
+    }
+
+    /// <inheritdoc />
+    public override bool CanRead => true;
+
+    /// <inheritdoc />
+    public override bool CanSeek => true;
+
+    /// <inheritdoc />
+    public override bool CanWrite => false;
+
+    /// <inheritdoc />
+    public override long Length => EnsureBuffer().Length;
+
+    /// <inheritdoc />
+    public override long Position
+    {
+        get => EnsureBuffer().Position;
+        set => EnsureBuffer().Position = value;
+    }
+
+    /// <inheritdoc />
+    public override void Flush() => _buffer?.Flush();
+
+    /// <inheritdoc />
+    public override int Read(byte[] buffer, int offset, int count) =>
+        EnsureBuffer().Read(buffer, offset, count);
+
+    /// <inheritdoc />
+    public override long Seek(long offset, SeekOrigin origin) =>
+        EnsureBuffer().Seek(offset, origin);
+
+    /// <inheritdoc />
+    public override void SetLength(long value) =>
+        throw new System.NotSupportedException("Stream does not support writing.");
+
+    /// <inheritdoc />
+    public override void Write(byte[] buffer, int offset, int count) =>
+        throw new System.NotSupportedException("Stream does not support writing.");
+}

PowerKit/MemoryWriteStream.cs

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+using System.IO;
+
+namespace PowerKit;
+
+/// <summary>
+/// A <see cref="Stream" /> wrapper that buffers all writes in memory and flushes them to the
+/// underlying stream on <see cref="Flush" />.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Writes go to an in-memory buffer and do not touch the underlying stream until
+/// <see cref="Flush" /> is called. This makes the wrapper always seekable and allows writes to
+/// be reordered freely before the final flush.
+/// </para>
+/// <para>
+/// On <see cref="Flush" />, the entire in-memory buffer is written to the underlying stream
+/// starting at its current position.
+/// </para>
+/// </remarks>
+public sealed class MemoryWriteStream(Stream source) : Stream
+{
+    private readonly MemoryStream _buffer = new();
+
+    /// <inheritdoc />
+    public override bool CanRead => false;
+
+    /// <inheritdoc />
+    public override bool CanSeek => true;
+
+    /// <inheritdoc />
+    public override bool CanWrite => true;
+
+    /// <inheritdoc />
+    public override long Length => _buffer.Length;
+
+    /// <inheritdoc />
+    public override long Position
+    {
+        get => _buffer.Position;
+        set => _buffer.Position = value;
+    }
+
+    /// <inheritdoc />
+    public override void Flush()
+    {
+        _buffer.Position = 0;
+        _buffer.CopyTo(source);
+        source.Flush();
+    }
+
+    /// <inheritdoc />
+    public override int Read(byte[] buffer, int offset, int count) =>
+        throw new System.NotSupportedException("Stream does not support reading.");
+
+    /// <inheritdoc />
+    public override long Seek(long offset, SeekOrigin origin) => _buffer.Seek(offset, origin);
+
+    /// <inheritdoc />
+    public override void SetLength(long value) => _buffer.SetLength(value);
+
+    /// <inheritdoc />
+    public override void Write(byte[] buffer, int offset, int count) =>
+        _buffer.Write(buffer, offset, count);
+}