Let ReadOnlySequenceReadBuffer be constructed over a ReadOnlySpan - #8

Follow-up from the MessagePack for C# v4 reader work. MessagePackReader wants ReadOnlySequenceReadBuffer as its only buffer field, but then it could not take span input, including stackalloc'd memory, because a span cannot be wrapped in a ReadOnlySequence<byte>.

Change

• New ReadOnlySequenceReadBuffer(ReadOnlySpan<byte> span). The span is served as a single window and the internal sequence stays empty. Nothing is ever stitched or rented, so there is no scratch overload.
• CommitAndReposition (the NoInlining slow path) gets one branch at the top: when sequence is empty, commit into the window instead of slicing the sequence.
  • An empty sequence already meant "the window holds every remaining byte" for a sequence read to the end. There the window is empty and nothing is pending, since Advance cannot pass the data, so the branch returns the same empty window as before.
  • Without the branch, a span-backed buffer asked for more than remains (TryGetSpan returning false) would throw from Slice on the empty sequence and lose the unread bytes.
• CopyToSlow is unreachable for span-backed buffers: their window is all remaining data, so the fast path covers every length the guard allows. A comment notes this.
• Hot paths are unchanged. Sequence input pays one IsEmpty check per segment crossing.
• CompatibleReadOnlySequenceReadBuffer is unchanged. It cannot hold a span, and memory input already works there via new ReadOnlySequence<byte>(memory).
• README: a short note under Read Buffers. It says ReadOnlySpanReadBuffer remains the smaller choice when input is always contiguous.

Tests

• The span-backed buffer is added to the existing every-read-buffer contract tests: Advance bounds, CopyTo (contiguous, straddling, past the data), and negative sizeHint.
• New ReadOnlySequenceReadBufferSpanTest covers:
  • the whole span as one window
  • sequential reads
  • a short demand that returns false and keeps the remaining bytes readable afterward (the regression case)
  • exhaustion and an empty span
  • one [NonCopyable] reader type taking both stackalloc and sequence input
• 156/156 pass locally.

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