Add polyfills for System.Numerics.BitOperations

[Copilot]

BitOperations had no polyfill coverage. Its members were introduced across four different .NET versions, so a single guard wasn't enough — the type itself doesn't exist before .NET Core 3.0, with individual members added later.

Version breakdown

• PolyShim/NetCore30/BitOperations.cs — defines the full type (PopCount, LeadingZeroCount, Log2, TrailingZeroCount, RotateLeft, RotateRight) for targets that predate .NET Core 3.0, since BitOperations isn't part of netstandard at any version either.
• PolyShim/Net60/BitOperations.cs — adds IsPow2 and RoundUpToPowerOf2 via extension members.
• PolyShim/Net70/BitOperations.cs — adds IntPtr/UIntPtr overloads of the above, matching the native nint/nuint overloads added that version.
• PolyShim/Net80/BitOperations.cs — adds the Crc32C overloads.

Each file is guarded to only compile in when the corresponding native members aren't already available, following the same "define type, then extend via later directories" pattern already used for RandomNumberGenerator.

// Works uniformly regardless of target framework
BitOperations.PopCount(0b_1011u);       // .NET Core 3.0+
BitOperations.IsPow2(1024);             // .NET 6.0+
BitOperations.LeadingZeroCount((UIntPtr)1); // .NET 7.0+
BitOperations.Crc32C(0, (byte)0x42);    // .NET 8.0+

Other changes

• Added BitOperationsTests.cs under the four corresponding PolyShim.Tests directories.
• Regenerated Signatures.md to include the new type and members.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.20%. Comparing base (24e0464) to head (e374b3f).

Additional details and impacted files

@@            Coverage Diff             @@
##            prime     #137      +/-   ##
==========================================
+ Coverage   99.17%   99.20%   +0.03%     
==========================================
  Files         150      154       +4     
  Lines        4005     4170     +165     
  Branches       88       89       +1     
==========================================
+ Hits         3972     4137     +165     
  Misses         13       13              
  Partials       20       20              

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[Tyrrrz]

@copilot please merge latest prime and regenerate Signatures.md

[Copilot]

@copilot please merge latest prime and regenerate Signatures.md

Merged prime and regenerated Signatures.md in b1da8c7/e374b3f.