Add BinaryPrimitives polyfills

[Copilot]

System.Buffers.Binary.BinaryPrimitives was introduced in .NET Core 2.1 / .NET Standard 2.1 and is unavailable on .NET Framework, older .NET Standard, and .NET Core < 2.1. This adds full polyfill coverage.

Changes

• PolyShim/NetCore21/BinaryPrimitives.cs — Full static class polyfill active under (NETCOREAPP && !NETCOREAPP2_1_OR_GREATER) || (NETSTANDARD && !NETSTANDARD2_1_OR_GREATER) || NETFRAMEWORK:

  • ReverseEndianness — short, ushort, int, uint, long, ulong
  • ReadXxx{Big,Little}Endian / TryReadXxx — Int16, UInt16, Int32, UInt32, Int64, UInt64, Single, Double
  • WriteXxx{Big,Little}Endian / TryWriteXxx — same types
  • Float bit-reinterpretation uses unsafe pointer cast when ALLOW_UNSAFE_BLOCKS is defined; falls back to BitConverter otherwise

• PolyShim/Net50/BinaryPrimitives.cs — Extension methods for ReverseEndianness(byte) and ReverseEndianness(sbyte) added in .NET 5, covering the gap on netcoreapp2.1–3.x and netstandard2.1

• PolyShim.Tests/NetCore21/BinaryPrimitivesTests.cs — 39 tests covering read/write round-trips, endianness correctness, and TryXxx boundary checks

// All of these now work on .NET Framework / netstandard2.0 / old netcoreapp
BinaryPrimitives.WriteInt32BigEndian(buf, 0x12345678);   // [0x12, 0x34, 0x56, 0x78]
BinaryPrimitives.ReadDoubleLittleEndian(bytes);           // 1.0
BinaryPrimitives.ReverseEndianness(0x0102030405060708L); // 0x0807060504030201L
BinaryPrimitives.TryWriteUInt64BigEndian(span, value);   // false if span too short

[Copilot]

Pull request overview

Adds a System.Buffers.Binary.BinaryPrimitives polyfill so consumers targeting .NET Framework, older .NET Standard, and older .NET Core can use the same BinaryPrimitives APIs (plus later-added byte/sbyte overloads) with accompanying tests and signature index updates.

Changes:

• Added a full BinaryPrimitives type polyfill for pre-.NET Core 2.1 / pre-.NET Standard 2.1 / .NET Framework targets.
• Added Net50 “member polyfills” for ReverseEndianness(byte) / ReverseEndianness(sbyte) (added in .NET 5).
• Added a new NetCore21 test suite covering common read/write/try patterns and endianness correctness; updated signature inventory.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
Signatures.md	Updates generated signature counts and adds BinaryPrimitives entries.
PolyShim/NetCore21/BinaryPrimitives.cs	New full polyfill implementation for BinaryPrimitives (reverse endianness + read/write + try APIs).
PolyShim/Net50/BinaryPrimitives.cs	Adds missing byte/sbyte ReverseEndianness members via member-polyfill extensions for pre-.NET 5 targets.
PolyShim.Tests/NetCore21/BinaryPrimitivesTests.cs	New tests for key endianness and try-read/write behaviors.

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[Tyrrrz]

@copilot please regenerated the Signatures.md file, as I've merged in the latest prime

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.19%. Comparing base (34a338c) to head (f26c3a3).

Additional details and impacted files

@@            Coverage Diff             @@
##            prime     #130      +/-   ##
==========================================
+ Coverage   99.16%   99.19%   +0.02%     
==========================================
  Files         147      148       +1     
  Lines        3846     3981     +135     
  Branches       87       87              
==========================================
+ Hits         3814     3949     +135     
  Misses         13       13              
  Partials       19       19              

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[Copilot]

@copilot please regenerated the Signatures.md file, as I've merged in the latest prime

Done! Regenerated Signatures.md in f26c3a3. The file now shows 620 total signatures (114 types, 506 members), up from 545, reflecting the BinaryPrimitives polyfills and changes from the latest prime branch.