Fix SIGBUS crash on ARMv7: use Unsafe.ReadUnaligned in Frozen.Hashing

[Copilot]

System.Collections.Frozen.Hashing was casting char* to uint* and dereferencing directly, causing unaligned 32-bit reads. On ARMv7 (e.g., Android Unity IL2CPP), the OS does not fixup misaligned accesses, resulting in SIGBUS crashes when calling ToFrozenSet<string>(StringComparer.Ordinal) on strings with length ≥ 4.

Changes

• Hashing.GetHashCodeOrdinal — replaced ((uint*)src)[n] and uint* ptrUInt32 loop with Unsafe.ReadUnaligned<uint>(ptr) / Unsafe.ReadUnaligned<uint>(ptr + 2)
• Hashing.GetHashCodeOrdinalIgnoreCaseAscii — same fix applied; same unaligned access pattern existed independently
• Added using System.Runtime.CompilerServices for Unsafe

Before (both methods, case 4 and default):

// Crashes on ARMv7 when src is not 4-byte aligned
hash1 = ... ^ ((uint*)src)[0];
hash2 = ... ^ ((uint*)src)[1];

uint* ptrUInt32 = (uint*)src;
while (length >= 4)
{
    hash1 = ... ^ ptrUInt32[0];
    hash2 = ... ^ ptrUInt32[1];
    ptrUInt32 += 2;
    ...
}

After:

hash1 = ... ^ Unsafe.ReadUnaligned<uint>(src);
hash2 = ... ^ Unsafe.ReadUnaligned<uint>(src + 2);

char* ptr = src;
while (length >= 4)
{
    hash1 = ... ^ Unsafe.ReadUnaligned<uint>(ptr);
    hash2 = ... ^ Unsafe.ReadUnaligned<uint>(ptr + 2);
    ptr += 4;
    ...
}

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[Copilot]

Pull request overview

Fixes a SIGBUS crash on ARMv7 caused by unaligned 32-bit reads in System.Collections.Frozen string hashing, by switching from char*→uint* dereferences to explicit unaligned reads via Unsafe.ReadUnaligned<uint>.

Changes:

• Replaced unaligned uint* dereferences in Hashing.GetHashCodeOrdinal with Unsafe.ReadUnaligned<uint> to safely read 32-bit chunks from potentially unaligned char*.
• Applied the same unaligned-read fix to Hashing.GetHashCodeOrdinalIgnoreCaseAscii.
• Added using System.Runtime.CompilerServices; for Unsafe.