Add ReadOnlySpan constructors to BitArray

[joshuajyue]

Summary

• Add ReadOnlySpan<bool>, ReadOnlySpan<byte>, and ReadOnlySpan<int> constructors to BitArray.
• Route the existing array constructors through the new span overloads while preserving null and overflow behavior.
• Preserve Boolean SIMD packing, scalar remainder handling, and integer endianness behavior.
• Add coverage for parity, slices, stack-allocated spans, source independence, SIMD boundaries, and noncanonical Boolean values.

Fixes #80263

Validation

• Checked CoreCLR and libraries baseline build
• Checked CoreLib rebuild and testhost refresh
• 216 BitArray_CtorTests passed with default hardware intrinsics
• 216 BitArray_CtorTests passed with DOTNET_EnableAVX2=0
• 216 BitArray_CtorTests passed with DOTNET_EnableHWIntrinsic=0
• 33,944 System.Collections tests passed

Performance

Full BenchmarkDotNet results from Windows ARM64 Release:

• Direct span construction was 33–51% faster than the existing span.ToArray() workaround across the measured sizes.
• Allocations were approximately 50% lower for byte and integer inputs and up to 89% lower for Boolean inputs.
• Existing array-constructor allocations were unchanged, with no observed regression for medium or large inputs.
• At 32 elements, delegating the existing Boolean and byte array constructors added approximately 1 ns. If that overhead is considered significant, those constructors could retain dedicated implementations at the cost of duplicated logic.

Note

This pull request description was generated with GitHub Copilot.

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[Copilot]

Pull request overview

Adds new ReadOnlySpan<T>-based constructors to System.Collections.BitArray to enable allocation-free construction from span-backed inputs, while preserving existing array-constructor behaviors by routing them through the new span overloads. This extends the public System.Collections contract and provides test coverage for span-specific scenarios (slices, stackalloc, copy semantics, and SIMD boundary safety).

Changes:

• Added BitArray(ReadOnlySpan<bool>), BitArray(ReadOnlySpan<byte>), and BitArray(ReadOnlySpan<int>) constructors and routed existing array constructors through them.
• Updated the System.Collections reference assembly to expose the new public constructors.
• Expanded unit tests to validate span parity with array constructors, slicing/stackalloc usage, source independence, non-canonical bool handling, and vector boundary safety.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Private.CoreLib/src/System/Collections/BitArray.cs	Implements the new span constructors and reuses them from existing array constructors without changing established behaviors (null/overflow handling, packing semantics).
src/libraries/System.Collections/tests/BitArray/BitArray_CtorTests.cs	Adds coverage for span constructors (slices, stackalloc, copy semantics, non-canonical bools, and BoundedMemory OOB checks).
src/libraries/System.Collections/ref/System.Collections.cs	Adds the new public constructor signatures to the ref surface for System.Collections.BitArray.