Add [NonCopyable] for SF002 and make ReadOnlySequenceReadBuffer's sequence parameter scoped - #7

Two changes requested by the MessagePack for C# v4 work, which is building a reader and a writer that own a SerializerFoundation buffer.

[NonCopyable] for SF002

A serializer's reader or writer is not a buffer, but it holds one by value. Copying it duplicates the inner buffer: two write indexes over one window, and pooled arrays returned twice at Dispose. SF002 could not see this because it only keys on IWriteBuffer / IReadBuffer.

• Adds SerializerFoundation.CodeAnalysis.NonCopyableAttribute (AttributeTargets.Struct), next to [RequireOverride].
• SF002 now treats a struct marked with it like a buffer struct. Every existing copy rule applies: by-value and in parameters, assignment, boxing, tuples, foreach, patterns, and so on.
• The analyzer matches this library's attribute type, not the name, so a same-named attribute from another library is ignored.
• The attribute is read from the generic definition, so marking Writer<TBuffer> covers every construction.
• Buffers still need no attribute; implementing the interface remains the marker.
• The diagnostic message now reads "a buffer, or marked [NonCopyable]". The rule ID, category and severity are unchanged, so the release tracking files don't change.

scoped on the ReadOnlySequenceReadBuffer constructors

Both constructors copy the sequence and never keep a reference to it. Without scoped, though, an in parameter on a ref struct constructor tells the compiler the result may capture that reference. Storing a buffer built from a by-value parameter or a local into a field then failed with CS8347, and callers had to work around it by passing the reference through MemoryMarshal.CreateReadOnlySpan.

• Both overloads (with and without scratch) now take scoped in ReadOnlySequence<byte>.
• CompatibleReadOnlySequenceReadBuffer is unchanged. It is not a ref struct, so its in parameter can't be captured.
• ScopedRefAttribute is emitted for netstandard2.0, netstandard2.1 and net9.0.

Notes for review

• ReadOnlySequenceReadBufferScopedTest is a compile-time regression check. With scoped removed, the file fails to build with CS8347 (verified locally). It also uses [NonCopyable] through the real analyzer.
• This adds public API (the attribute), so the next release should probably be a minor version bump.
• Tests: 150/150 pass locally.

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