Move Zstandard APIs from System.IO.Compression.Zstandard to System.IO.Compression

[rzikm]

There is no real benefit from having the APIs in separate assembly because:

• the Managed code size is rather small
• it P/Invokes System.IO.Compression.Native just like System.IO.Compression managed assembly does
• It would lead to circular dependency if we decide to implement Support Zstandard compression in ZipArchives #123532

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[Copilot]

Pull request overview

This PR consolidates Zstandard compression APIs into the main System.IO.Compression library, removing the separate System.IO.Compression.Zstandard assembly and updating build/test infrastructure accordingly.

Changes:

• Move Zstandard implementation and tests under System.IO.Compression (including ref API surface updates).
• Add browser/WASI PlatformNotSupportedException stubs while keeping full implementations on Windows/Unix TFMs.
• Remove the old System.IO.Compression.Zstandard project artifacts and references (solutions, props, project references).

Reviewed changes

Copilot reviewed 23 out of 31 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/libraries/sfx.slnx	Removes System.IO.Compression.Zstandard ref project from the libraries solution.
src/libraries/System.Net.Http/src/System.Net.Http.csproj	Drops project reference to the removed System.IO.Compression.Zstandard project.
src/libraries/System.IO.Compression/tests/Zstandard/ZstandardTestUtils.cs	Adds shared helpers for Zstandard tests in the consolidated test project.
src/libraries/System.IO.Compression/tests/Zstandard/ZstandardEncoderDecoderTests.cs	Adds encoder/decoder coverage to System.IO.Compression tests.
src/libraries/System.IO.Compression/tests/Zstandard/ZstandardDictionaryTests.cs	Adds dictionary creation/training tests under System.IO.Compression.
src/libraries/System.IO.Compression/tests/Zstandard/ZstandardCompressionOptionsTests.cs	Adds validation tests for ZstandardCompressionOptions.
src/libraries/System.IO.Compression/tests/Zstandard/CompressionStreamUnitTests.Zstandard.cs	Adds ZstandardStream coverage integrated with existing stream test base.
src/libraries/System.IO.Compression/tests/System.IO.Compression.Tests.csproj	Integrates Zstandard test sources conditionally (excluding browser/WASI).
src/libraries/System.IO.Compression/src/System/IO/Compression/Zstandard/ZstandardUtils.cs	Adds shared internal helpers for Zstandard implementation.
src/libraries/System.IO.Compression/src/System/IO/Compression/Zstandard/ZstandardStream.cs	Adds platform attributes on ZstandardStream.
src/libraries/System.IO.Compression/src/System/IO/Compression/Zstandard/ZstandardStream.Decompress.cs	Adds decompression-side stream implementation under the consolidated assembly.
src/libraries/System.IO.Compression/src/System/IO/Compression/Zstandard/ZstandardStream.Compress.cs	Adds compression-side stream implementation under the consolidated assembly.
src/libraries/System.IO.Compression/src/System/IO/Compression/Zstandard/ZstandardEncoder.cs	Adds platform attributes on ZstandardEncoder.
src/libraries/System.IO.Compression/src/System/IO/Compression/Zstandard/ZstandardDictionary.cs	Adds platform attributes on ZstandardDictionary.
src/libraries/System.IO.Compression/src/System/IO/Compression/Zstandard/ZstandardDecoder.cs	Adds platform attributes on ZstandardDecoder.
src/libraries/System.IO.Compression/src/System/IO/Compression/Zstandard/ZstandardCompressionOptions.cs	Adds platform attributes on ZstandardCompressionOptions.
src/libraries/System.IO.Compression/src/System/IO/Compression/Zstandard/Zstandard.PlatformNotSupported.cs	Adds browser/WASI stubs throwing PlatformNotSupportedException.
src/libraries/System.IO.Compression/src/System.IO.Compression.csproj	Multi-target integration for Zstandard sources + stubs under System.IO.Compression.
src/libraries/System.IO.Compression/src/Resources/Strings.resx	Merges Zstandard resource strings into System.IO.Compression resources.
src/libraries/System.IO.Compression/ref/System.IO.Compression.cs	Adds Zstandard APIs to the System.IO.Compression reference surface with platform attributes.
src/libraries/System.IO.Compression/README.md	Updates README to reflect Zstandard support in System.IO.Compression.
src/libraries/System.IO.Compression/Directory.Build.props	Removes BOM/normalizes file header.
src/libraries/System.IO.Compression.Zstandard/tests/System.IO.Compression.Zstandard.Tests.csproj	Deletes old Zstandard test project (now integrated into System.IO.Compression tests).
src/libraries/System.IO.Compression.Zstandard/src/System.IO.Compression.Zstandard.csproj	Deletes old Zstandard implementation project (now merged into System.IO.Compression).
src/libraries/System.IO.Compression.Zstandard/src/Resources/Strings.resx	Deletes old resource file (strings moved to System.IO.Compression).
src/libraries/System.IO.Compression.Zstandard/ref/System.IO.Compression.Zstandard.csproj	Deletes old ref project.
src/libraries/System.IO.Compression.Zstandard/ref/System.IO.Compression.Zstandard.cs	Deletes old ref surface file (moved into System.IO.Compression).
src/libraries/System.IO.Compression.Zstandard/System.IO.Compression.Zstandard.slnx	Deletes old solution for the removed library.
src/libraries/System.IO.Compression.Zstandard/README.md	Deletes old library README (functionality now documented under System.IO.Compression).
src/libraries/System.IO.Compression.Zstandard/Directory.Build.props	Deletes old build props (no longer needed after consolidation).
src/libraries/NetCoreAppLibrary.props	Removes System.IO.Compression.Zstandard from the NetCoreApp library list.

[Copilot]

Pull request overview

Copilot reviewed 23 out of 31 changed files in this pull request and generated 1 comment.

[iremyux]

LGTM