Add BinaryReader/BinaryWriter span polyfills (NetCore21)

[Copilot]

Adds polyfills for the four span-based BinaryReader/BinaryWriter APIs introduced in .NET Core 2.1, enabling their use on netstandard2.0, netframework, and older netcoreapp targets.

New polyfills

• BinaryReader.Read(Span<byte>) — delegates to Read(byte[], int, int), copies only bytes actually read
• BinaryReader.Read(Span<char>) — delegates to Read(char[], int, int), copies only chars actually read
• BinaryWriter.Write(ReadOnlySpan<byte>) — delegates to Write(byte[], int, int)
• BinaryWriter.Write(ReadOnlySpan<char>) — delegates to Write(char[], int, int)

using var stream = new MemoryStream([1, 2, 3, 4, 5]);
using var reader = new BinaryReader(stream);
Span<byte> buffer = stackalloc byte[5];
var bytesRead = reader.Read(buffer); // polyfilled on pre-2.1 targets

using var output = new MemoryStream();
using var writer = new BinaryWriter(output);
writer.Write((ReadOnlySpan<byte>)buffer); // polyfilled on pre-2.1 targets

[Copilot]

Pull request overview

Adds .NET Core 2.1 span-based BinaryReader/BinaryWriter API polyfills so older targets (e.g., netstandard2.0, netframework, pre-2.1 netcoreapp) can use the newer span overloads while delegating to the existing array-based methods.

Changes:

• Add BinaryReader.Read(Span<byte>) and BinaryReader.Read(Span<char>) polyfills (delegate to array overloads and copy back only what was read).
• Add BinaryWriter.Write(ReadOnlySpan<byte>) and BinaryWriter.Write(ReadOnlySpan<char>) polyfills (delegate to array overloads).
• Add unit tests for the new polyfills and update Signatures.md to list the new APIs.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
Signatures.md	Adds the BinaryReader/BinaryWriter span-based members to the signature index and updates totals.
PolyShim/NetCore21/BinaryWriter.cs	Introduces BinaryWriter.Write(ReadOnlySpan<byte/char>) member polyfills for pre-.NET Core 2.1 targets.
PolyShim/NetCore21/BinaryReader.cs	Introduces BinaryReader.Read(Span<byte/char>) member polyfills for pre-.NET Core 2.1 targets.
PolyShim.Tests/NetCore21/BinaryWriterTests.cs	Adds tests for the new BinaryWriter span overload polyfills.
PolyShim.Tests/NetCore21/BinaryReaderTests.cs	Adds tests for the new BinaryReader span overload polyfills.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.16%. Comparing base (1f7fe37) to head (8d1ada6).

Additional details and impacted files

@@            Coverage Diff             @@
##            prime     #131      +/-   ##
==========================================
+ Coverage   99.15%   99.16%   +0.01%     
==========================================
  Files         145      147       +2     
  Lines        3798     3846      +48     
  Branches       87       87              
==========================================
+ Hits         3766     3814      +48     
  Misses         13       13              
  Partials       19       19              

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