Add polyfills for BigInteger.ToByteArray overloads and related .NET Core 2.1 members

[Copilot]

BigInteger gained a byte-array overload family in .NET Core 2.1 / .NET Standard 2.1 (ToByteArray(bool, bool), GetByteCount(bool), TryWriteBytes(Span<byte>, out int, bool, bool)) that wasn't available as a polyfill for older target frameworks.

Polyfill implementation

• Added PolyShim/NetCore21/BigInteger.cs with extension members for:
  • ToByteArray(bool isUnsigned, bool isBigEndian)
  • GetByteCount(bool isUnsigned)
  • TryWriteBytes(Span<byte> destination, out int bytesWritten, bool isUnsigned, bool isBigEndian)
• Built on top of the native parameterless ToByteArray(), handling sign-byte trimming for unsigned output, endianness reversal, and throwing OverflowException for negative values when isUnsigned: true, matching documented BCL behavior.
• Guarded with a nested #if to exclude frameworks where BigInteger itself doesn't exist (net35, netstandard1.0), in addition to the standard upper-bound guard for frameworks that already have these members natively.

BigInteger value = 33022;

value.ToByteArray(isUnsigned: true, isBigEndian: true); // { 0x80, 0xFE }
value.GetByteCount(isUnsigned: true);                   // 2
value.TryWriteBytes(destination, out var written, isUnsigned: true, isBigEndian: true);

Not polyfilled

• The BigInteger(ReadOnlySpan<byte>, bool, bool) constructor overload (also added in .NET Core 2.1) can't be polyfilled — C# extension members can't add constructors to an existing type.

Tests & docs

• Added PolyShim.Tests/NetCore21/BigIntegerTests.cs covering signed/unsigned and little/big-endian combinations, zero value, negative overflow, and TryWriteBytes success/insufficient-buffer paths.
• Regenerated Signatures.md.

[codecov]

Codecov Report

❌ Patch coverage is 98.63014% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.20%. Comparing base (87ebebc) to head (9178814).

Files with missing lines	Patch %	Lines
PolyShim/NetCore21/BigInteger.cs	95.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            prime     #152      +/-   ##
==========================================
+ Coverage   88.17%   88.20%   +0.03%     
==========================================
  Files         349      351       +2     
  Lines        7821     7894      +73     
  Branches      558      562       +4     
==========================================
+ Hits         6896     6963      +67     
- Misses        755      759       +4     
- Partials      170      172       +2     

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[Copilot]

Copilot review overview

🟢 Approval recommended

No blocking issues were identified in the implementation, compatibility guards, tests, or signature updates.

Review effort: Balanced
Findings: None

What changed in this PR

Adds three BigInteger byte-conversion polyfills for older target frameworks, extending .NET Core 2.1 API compatibility.

Changes:

• Implements byte conversion, byte counting, and span writing with signedness and endianness support.
• Adds tests for conversion options, overflow, and buffer handling.
• Updates the signature catalog.

File	Description
Signatures.md	Lists the three new members and updates totals.
PolyShim/​NetCore21/​BigInteger.cs	Implements the polyfills with framework guards.
PolyShim.Tests/​NetCore21/​BigIntegerTests.cs	Tests conversion behavior and buffer handling.

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