Extract StreamPortal utility from Ressy into PowerKit

[Copilot]

Ports StreamPortal from Ressy into PowerKit as a first-class public utility.

Changes

• StreamPortal.cs — new public class that captures a stream position and exposes Jump(), which seeks to that position and returns an IDisposable that restores the original position on dispose. Constructor guards against non-seekable streams and negative positions.
• StreamExtensions.cs — two new CreatePortal extension methods on Stream: one targeting the current position, one accepting an explicit offset.

Usage

using var stream = File.OpenRead("data.bin");
stream.Seek(100, SeekOrigin.Begin);

var portal = stream.CreatePortal(); // saves position 100

// later...
using (portal.Jump())
{
    // stream is at 100 here
    var header = reader.ReadBytes(16);
} // stream restored to wherever it was before Jump()

[Tyrrrz]

These two tests can be merged @copilot

[Copilot]

Done — merged the two tests into Jump_SeeksToPortalPositionAndRestoresOnDispose_Test in commit Merge Jump_SeeksToPortalPosition and Jump_RestoresPositionOnDispose tests into one. The single test now asserts both that Jump() seeks to the portal position and that Dispose() restores the original position.

[Copilot]

Pull request overview

Ports the StreamPortal concept into PowerKit to provide a reusable utility for saving a stream position and temporarily jumping back to it with automatic restoration via IDisposable.

Changes:

• Added StreamPortal public utility type for capturing a stream position and restoring it after a scoped jump.
• Added Stream.CreatePortal() / Stream.CreatePortal(long) extension methods for convenient portal creation.
• Added unit tests validating basic portal creation and jump/restore behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
PowerKit/StreamPortal.cs	Introduces the StreamPortal type with position capture, validation, and Jump() restore semantics.
PowerKit/Extensions/StreamExtensions.cs	Adds CreatePortal extension methods to construct StreamPortal instances from a Stream.
PowerKit.Tests/StreamPortalTests.cs	Adds tests for portal creation and jump/restore behavior.

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[codecov]

Codecov Report

❌ Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.43%. Comparing base (7501f41) to head (c71284c).

Files with missing lines	Patch %	Lines
PowerKit/StreamPortal.cs	92.30%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            prime      #89      +/-   ##
==========================================
+ Coverage   86.35%   86.43%   +0.08%     
==========================================
  Files          74       75       +1     
  Lines        1253     1268      +15     
  Branches      234      235       +1     
==========================================
+ Hits         1082     1096      +14     
  Misses        121      121              
- Partials       50       51       +1     

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