Replace unsafe code with safe APIs in CoreLib and libraries

[EgorBo]

Replace selected unsafe operations with safe span, indexing, and SIMD APIs in CoreLib and networking.

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[EgorBo]

@MihuBot

[Copilot]

Copilot review overview

🟡 Changes recommended

The StreamWriter and SharedArrayPool hot-path substitutions need benchmark evidence demonstrating that they do not restore known overhead.

Review effort: Balanced
Findings: 2

Open (2)

• Avoid covariance checks in SharedArrayPool return path · New
• Restore benchmarks before using span copy in WriteSpan · New

What changed in this PR

Replaces selected unsafe pointer/reference operations with safe span, indexing, endianness, and SIMD APIs across CoreLib, networking, and XML.

Changes:

• Converts pointer-based buffer and string operations to spans and indexing.
• Reworks SIMD/table accesses using safe vector and binary APIs.
• Removes unsafe code from WebSocket masking and socket byte operations.

Two hot-path substitutions require performance validation before approval.

File	Description
XmlCharType.cs	Uses indexed character-property lookup.
UTF8Encoding.Sealed.cs	Uses stack-allocated spans for small conversions.
String.Searching.cs	Uses string span search APIs.
String.Manipulation.cs	Converts trimming helper to ReadOnlySpan<char>.
StringSearchValuesHelper.cs	Uses safe vector and scalar reads.
IndexOfAnyAsciiSearcher.cs	Replaces pointer bitmap mutation with vector APIs.
BitmapCharSearchValues.cs	Uses span-based iteration.
StreamWriter.cs	Replaces pointer copying with span copying.
CharUnicodeInfo.cs	Uses indexed and endian-aware table reads.
SharedArrayPool.cs	Replaces indirect reference storage with array indexing.
BinaryPrimitives.ReverseEndianness.cs	Uses span-based vector loads and stores.
BitConverter.cs	Uses direct byte indexing for Boolean conversion.
ManagedWebSocket.cs	Implements masking with safe scalar and vector APIs.
NetworkStream.cs	Uses single-element spans for byte operations.

[EgorBo]

@MihuBot

[EgorBo]

@MihaZupan the current subset seems to be bound check free (only some minor extra instructions or heavier encodings)

[MihaZupan]

I think this will add an extra bounds check in the loop
https://godbolt.org/z/n9a99Tjn1

[EgorBo]

hm.. let me see if it's fixable in JIT. The loop looks trivial enough

[EgorBo]

@MihaZupan it seems like it's more or less trivial to remove in jit, will do there 🙂 #135008