Add Memory<byte> and ReadOnlyMemory<byte> constructors to MemoryStream

[Copilot]

Description

Adds ReadOnlyMemory<byte> and Memory<byte> constructors to MemoryStream using a delegation pattern that avoids any performance penalty for existing byte-array code paths.

A private inner class MemoryMemoryStream holds all Memory-backed state and operations. A readonly field _memoryMemoryStream is only set by the new constructors — in all existing constructors it remains null. Every public/override method checks this field first:

public override int ReadByte()
{
    if (_memoryMemoryStream is not null)
        return _memoryMemoryStream.ReadByte();

    // existing byte-array implementation unchanged
    EnsureNotClosed();
    if (_position >= _length) return -1;
    return _buffer[_position++];
}

New constructors

• MemoryStream(ReadOnlyMemory<byte>) — read-only, non-expandable
• MemoryStream(Memory<byte>) — writable, non-expandable
• MemoryStream(Memory<byte>, bool writable) — writability control

Key design decisions

• Zero perf regression: Existing callers pay only one null-check per method call. No virtual dispatch, no interface indirection.
• BinaryReader compatibility: BinaryReader.ReadCharsCommon used InternalGetBuffer()/InternalGetPosition() which return byte[] — not viable for arbitrary Memory<byte>. Added InternalRead(int count) that returns ReadOnlySpan<byte> and works for both paths.
• Non-expandable: Memory-backed streams behave like MemoryStream(byte[], writable) — fixed capacity, SetLength within capacity is allowed, expansion throws NotSupportedException.
• GetBuffer/TryGetBuffer: GetBuffer() throws UnauthorizedAccessException, TryGetBuffer() returns false (no underlying byte[] to expose).
• ToArray after Dispose: Works, matching existing byte-array MemoryStream behavior.

Files changed

File	Change
MemoryStream.cs	Inner class, 3 constructors, delegation in all methods, InternalRead
BinaryReader.cs	Use InternalRead instead of InternalGetBuffer/InternalGetPosition
System.Runtime.cs	Ref API surface
MemoryStreamTests.cs	23 new tests

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[jkotas]

Store only ReadOnlyMemory/Memory here and use the rest of the state from the main Stream?

It would reduce the overall instance size and it would make it unnecessary to the switch in some of the properties, e.g. Length or Position.