bench(#141): add end-to-end serialisation benchmark

Add SerialisationBenchmark covering Encode, Decode, EncodeCompressed,
and DecodeCompressed with MemoryDiagnoser for allocation tracking.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: LeeCampbell

HdrHistogram.Benchmarking/Serialisation/SerialisationBenchmark.cs

@@ -0,0 +1,61 @@
+using BenchmarkDotNet.Attributes;
+using HdrHistogram.Encoding;
+
+namespace HdrHistogram.Benchmarking.Serialisation
+{
+    [MemoryDiagnoser]
+    public class SerialisationBenchmark
+    {
+        private LongHistogram _source = null!;
+        private Utilities.ByteBuffer _encodeBuffer = null!;
+        private Utilities.ByteBuffer _decodeBuffer = null!;
+        private Utilities.ByteBuffer _compressedDecodeBuffer = null!;
+
+        [GlobalSetup]
+        public void Setup()
+        {
+            _source = new LongHistogram(3600_000_000L, 3);
+            for (long i = 0; i < 10_000; i++)
+            {
+                _source.RecordValue(1000L * i);
+            }
+
+            var capacity = _source.GetNeededByteBufferCapacity();
+            _encodeBuffer = Utilities.ByteBuffer.Allocate(capacity);
+
+            // Pre-encode for uncompressed decode benchmark
+            _source.Encode(_decodeBuffer = Utilities.ByteBuffer.Allocate(capacity), HistogramEncoderV2.Instance);
+
+            // Pre-encode for compressed decode benchmark
+            _source.EncodeIntoCompressedByteBuffer(_compressedDecodeBuffer = Utilities.ByteBuffer.Allocate(capacity));
+        }
+
+        [Benchmark]
+        public int Encode()
+        {
+            _encodeBuffer.Position = 0;
+            return _source.Encode(_encodeBuffer, HistogramEncoderV2.Instance);
+        }
+
+        [Benchmark]
+        public HistogramBase Decode()
+        {
+            _decodeBuffer.Position = 0;
+            return HistogramEncoding.DecodeFromByteBuffer(_decodeBuffer, 0);
+        }
+
+        [Benchmark]
+        public int EncodeCompressed()
+        {
+            _encodeBuffer.Position = 0;
+            return _source.EncodeIntoCompressedByteBuffer(_encodeBuffer);
+        }
+
+        [Benchmark]
+        public HistogramBase DecodeCompressed()
+        {
+            _compressedDecodeBuffer.Position = 0;
+            return HistogramEncoding.DecodeFromCompressedByteBuffer(_compressedDecodeBuffer, 0);
+        }
+    }
+}