Add BoisBufferWriter for buffered BOIS output paths

[Copilot]

This adds a dedicated BOIS writer with an 8KB staged cache for output targets that previously used direct array/stream writers. The new writer keeps buffered state across write calls, exposes explicit flush semantics, and routes large payloads without extra staging when the cache would not help.

• New buffered writer

  • Adds Salar.Bois.Types.BoisBufferWriter : BufferWriterBase
  • Supports the existing constructor shapes used by BOIS serialization:
    • byte[]
    • byte[], int position, int length
    • Stream
  • Buffers writes into an internal 8 * 1024 cache
  • Exposes Flush() to publish buffered bytes to the final destination

• Hot-path write behavior

  • Keeps primitive writes on fixed-size span-based paths to minimize overhead
  • Flushes only when the cache is full or when explicitly requested
  • Writes large ReadOnlySpan<byte> payloads directly to the underlying array/stream instead of copying them through the cache first

• Serializer integration

  • Updates serializer overloads that construct writers internally to use BoisBufferWriter
  • Flushes before returning from serializer-managed array and stream outputs so callers see fully committed data

• Coverage

  • Adds focused tests for:
    • explicit Flush() behavior
    • stream-backed serialization beyond the 8KB cache
    • byte-array-backed serialization beyond the 8KB cache

using var stream = new MemoryStream();
var writer = new BoisBufferWriter(stream);

writer.Write(new byte[] { 1, 2, 3, 4 });
// bytes are still buffered here

writer.Flush();
// bytes are now committed to the stream