feat(network)!: add v2 framing, pooled buffers and bounded dispatch

Runtime/Serialization/ShrinkBufferWriter.cs

@@ -0,0 +1,40 @@
+#nullable enable
+using System;
+using System.Buffers;
+
+namespace ShrinkNetwork
+{
+    /// <summary>Owns rented memory until Dispose. A sender must await completion before disposing.</summary>
+    public sealed class ShrinkBufferWriter : IBufferWriter<byte>, IDisposable
+    {
+        private byte[]? _buffer;
+        public ShrinkBufferWriter(int initialCapacity = 256) => _buffer = ArrayPool<byte>.Shared.Rent(Math.Max(1, initialCapacity));
+        public int WrittenCount { get; private set; }
+        public ReadOnlyMemory<byte> WrittenMemory => Buffer.AsMemory(0, WrittenCount);
+        internal Span<byte> WrittenSpan => Buffer.AsSpan(0, WrittenCount);
+        private byte[] Buffer => _buffer ?? throw new ObjectDisposedException(nameof(ShrinkBufferWriter));
+        public void Advance(int count)
+        {
+            if (count < 0 || count > Buffer.Length - WrittenCount) throw new ArgumentOutOfRangeException(nameof(count));
+            WrittenCount += count;
+        }
+        public Memory<byte> GetMemory(int sizeHint = 0) { Ensure(sizeHint); return Buffer.AsMemory(WrittenCount); }
+        public Span<byte> GetSpan(int sizeHint = 0) { Ensure(sizeHint); return Buffer.AsSpan(WrittenCount); }
+        private void Ensure(int sizeHint)
+        {
+            if (sizeHint < 0) throw new ArgumentOutOfRangeException(nameof(sizeHint));
+            sizeHint = Math.Max(1, sizeHint);
+            if (sizeHint <= Buffer.Length - WrittenCount) return;
+            var next = ArrayPool<byte>.Shared.Rent(checked(Math.Max(Buffer.Length * 2, WrittenCount + sizeHint)));
+            Buffer.AsSpan(0, WrittenCount).CopyTo(next);
+            ArrayPool<byte>.Shared.Return(Buffer, clearArray: true);
+            _buffer = next;
+        }
+        public void Dispose()
+        {
+            var buffer = _buffer;
+            _buffer = null;
+            if (buffer != null) ArrayPool<byte>.Shared.Return(buffer, clearArray: true);
+        }
+    }
+}