Document breaking change: PackagePart.GetStream() returns non-seekable stream for compressed parts in ReadWrite packages

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System.IO.Packaging.PackagePart.GetStream() now returns a forward-only stream instead of a seekable MemoryStream when reading a compressed, unmodified part from a package opened with FileAccess.ReadWrite, following dotnet/runtime#129698. This change adds the corresponding breaking-change documentation.

New article

• Added docs/core/compatibility/core-libraries/11/packagepart-getstream-non-seekable.md covering:
  • Previous behavior (seekable MemoryStream) vs. new behavior (forward-only stream, CanSeek == false)
  • The five conditions required to observe the change, and scenarios that remain unaffected
  • Recommended action: manually copy to a MemoryStream when seekability is required, or use FileAccess.Read

Indexing

• Added the article to docs/core/compatibility/toc.yml under Core .NET libraries for .NET 11
• Added a row to the docs/core/compatibility/11.md breaking changes summary table

Example from the article

using Stream partStream = part.GetStream(FileMode.Open, FileAccess.Read);
using var seekable = new MemoryStream();
partStream.CopyTo(seekable);
seekable.Position = 0;
// Use 'seekable'. It's fully buffered and seekable.

• Fixes [Breaking change]: PackagePart.GetStream() returns a non-seekable stream for compressed parts in ReadWrite packages #54822

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Internal previews

File	Preview link
docs/core/compatibility/11.md	docs/core/compatibility/11
docs/core/compatibility/core-libraries/11/packagepart-getstream-non-seekable.md	docs/core/compatibility/core-libraries/11/packagepart-getstream-non-seekable
docs/core/compatibility/toc.yml	docs/core/compatibility/toc

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[adegeo]

I think this line is incorrect and needs to be removed. The intro to the list is talking about when you would see this change, the change being that a forward-only stream is returned. This line is actually describing what causes the exception to throw, which is just a artifact of trying to seek with a non-seekable stream. It has nothing to do with the change.

[gewarren]

The list is the steps required to observe the change. The change would still be there, you just wouldn't observe it, if you didn't seek or read Position. cc @rzikm for his thoughts.

[adegeo]

That's not entirely true though, if the logic checks stream.CanSeek before setting the position, no code will fail via an exception. However, the behavior may now have changed because the code is taking a different path. I think "observe" is the wrong word to use here. I viewed the list as what conditions need to exist to force the underlying type to change, which is what the breaking change is about.