Use safe span slice-advance loop in Number digit parser

[EgorBo]

Note

This PR was authored with assistance from GitHub Copilot.

Replaces the raw byte* + Unsafe.ReadUnaligned<ulong> SWAR digit-parsing path in Number.NumberToFloatingPointBits.cs with the safe span-based "slice-advance" loop pattern that the JIT now recognizes and bounds-check elides.

This is the same pattern PR #127381 (NumericsHelpers) and #127382 (HashCode) use:

while (data.Length >= CONST)
{
    ...
    data = data.Slice(CONST);
}

Changes:

• ParseEightDigitsUnrolled(byte*) → ParseEightDigitsUnrolled(ReadOnlySpan<byte>). Uses BinaryPrimitives.ReadUInt64LittleEndian directly, which removes the manual if (!BitConverter.IsLittleEndian) val = ReverseEndianness(val); branch on big-endian.
• DigitsToUInt32(byte*, int) / DigitsToUInt64(byte*, int) → DigitsToUInt32(ReadOnlySpan<byte>) / DigitsToUInt64(ReadOnlySpan<byte>) with the JIT-friendly while (digits.Length >= 8) { …; digits = digits.Slice(8); } chunked shape and a safe foreach byte tail.
• [RequiresUnsafe] removed from all three.
• Updated the two callers (AccumulateDecimalDigitsIntoBigInteger and the ≤19-digit fast path in NumberToFloatingPointBits<TFloat>) to construct a ReadOnlySpan<byte> over number.DigitsPtr.

Codegen diff: TODO

Validation: full System.Runtime.Tests run — only the same 8 unrelated pre-existing failures (TimeZoneInfo NearMaxValue/NearMinValue, UnionAttributes); 7089 Single/Double/Half/Decimal parse tests pass.

[EgorBo]

@MihuBot

[Copilot]

Pull request overview

This PR updates the low-level decimal digit parsing logic used by NumberToFloatingPointBits to avoid raw pointer-walking + Unsafe.ReadUnaligned in favor of ReadOnlySpan<byte> and a JIT-friendly while (span.Length >= CONST) { …; span = span.Slice(CONST); } loop shape, aiming to preserve bounds-check elision while removing [RequiresUnsafe] usage from the helpers.

Changes:

• Convert ParseEightDigitsUnrolled, DigitsToUInt32, and DigitsToUInt64 to ReadOnlySpan<byte>-based implementations.
• Update AccumulateDecimalDigitsIntoBigInteger and the ≤19-digit fast path in NumberToFloatingPointBits<TFloat> to construct ReadOnlySpan<byte> over number.DigitsPtr.
• Use BinaryPrimitives.ReadUInt64LittleEndian directly to handle endianness.

[EgorBo]

@MihuBot