Replace ArrayBuffer with anonymous mmap for Memory mode and stream construction

ArrayBuffer was backed by byte[], which is limited to ~2.1 GiB in .NET.
This adds a new MemoryBuffer class that uses anonymous (non-file-backed)
MemoryMappedFile, allowing FileAccessMode.Memory and the Reader(Stream)
constructor to handle databases larger than 2 GiB. ArrayBuffer is retained
only for small internal byte[] uses in TypeActivatorCreator and tests.

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

Author: horgh

CLAUDE.md

@@ -69,8 +69,9 @@ The library has three main architectural layers:
 
 1. **Buffer Layer** - File access abstraction
    - `Buffer` (abstract base): Defines read interface for binary data
-   - `MemoryMapBuffer`: Memory-mapped file implementation (default, best balance)
-   - `ArrayBuffer`: In-memory byte array implementation (fastest lookups, highest RAM usage)
+   - `MemoryMapBuffer`: Named memory-mapped file implementation (default, supports cross-process sharing)
+   - `MemoryBuffer`: Anonymous memory-mapped file implementation (used by `FileAccessMode.Memory` and stream construction)
+   - `ArrayBuffer`: In-memory byte array implementation (used internally for small buffers in `TypeActivatorCreator` and tests)
 
 2. **Reader Layer** - Database navigation and IP lookup
    - `Reader`: Main entry point for IP address lookups

MaxMind.Db.Test/PointerTest.cs

@@ -16,7 +16,7 @@ public void TestWithPointers()
         {
             var path = Path.Combine(TestUtils.TestDirectory, "TestData", "MaxMind-DB", "test-data", "maps-with-pointers.raw");
 
-            using var database = new ArrayBuffer(path);
+            using var database = new ArrayBuffer(File.ReadAllBytes(path));
             var decoder = new Decoder(database, 0);
 
             var node = decoder.Decode<Dictionary<string, object>>(0, out _);

MaxMind.Db/ArrayBuffer.cs

@@ -1,9 +1,7 @@
 #region
 
 using System;
-using System.IO;
 using System.Text;
-using System.Threading.Tasks;
 
 #endregion
 
@@ -19,73 +17,6 @@ public ArrayBuffer(byte[] array)
             _fileBytes = array;
         }
 
-        public ArrayBuffer(string file) : this(File.ReadAllBytes(file))
-        {
-        }
-
-        internal ArrayBuffer(Stream stream) : this(BytesFromStream(stream))
-        {
-        }
-
-        public static async Task<ArrayBuffer> CreateAsync(string file)
-        {
-            using var stream = new FileStream(file, FileMode.Open, FileAccess.Read, FileShare.Read, 4096, true);
-            return await CreateAsync(stream).ConfigureAwait(false);
-        }
-
-        internal static async Task<ArrayBuffer> CreateAsync(Stream stream)
-        {
-            return new ArrayBuffer(await BytesFromStreamAsync(stream).ConfigureAwait(false));
-        }
-
-        private static byte[] BytesFromStream(Stream stream)
-        {
-            if (stream == null)
-            {
-                throw new ArgumentNullException(nameof(stream), "The database stream must not be null.");
-            }
-
-            byte[] bytes;
-
-            using (var memoryStream = new MemoryStream())
-            {
-                stream.CopyTo(memoryStream);
-                bytes = memoryStream.ToArray();
-            }
-
-            if (bytes.Length == 0)
-            {
-                throw new InvalidDatabaseException(
-                    "There are zero bytes left in the stream. Perhaps you need to reset the stream's position.");
-            }
-
-            return bytes;
-        }
-
-        private static async Task<byte[]> BytesFromStreamAsync(Stream stream)
-        {
-            if (stream == null)
-            {
-                throw new ArgumentNullException(nameof(stream), "The database stream must not be null.");
-            }
-
-            byte[] bytes;
-
-            using (var memoryStream = new MemoryStream())
-            {
-                await stream.CopyToAsync(memoryStream).ConfigureAwait(false);
-                bytes = memoryStream.ToArray();
-            }
-
-            if (bytes.Length == 0)
-            {
-                throw new InvalidDatabaseException(
-                    "There are zero bytes left in the stream. Perhaps you need to reset the stream's position.");
-            }
-
-            return bytes;
-        }
-
         public override byte[] Read(long offset, int count)
         {
             var bytes = new byte[count];

MaxMind.Db/GlobalSuppressions.cs

@@ -10,7 +10,6 @@
 [assembly: SuppressMessage("Design", "CA1034:Nested types should not be visible", Justification = "<Pending>", Scope = "type", Target = "~T:MaxMind.Db.Reader.ReaderIteratorNode`1")]
 [assembly: SuppressMessage("Performance", "CA1815:Override equals and operator equals on value types", Justification = "<Pending>", Scope = "type", Target = "~T:MaxMind.Db.Reader.ReaderIteratorNode`1")]
 [assembly: SuppressMessage("Interoperability", "CA1416:Validate platform compatibility", Justification = "<Pending>", Scope = "member", Target = "~M:MaxMind.Db.MemoryMapBuffer.#ctor(System.String,System.Boolean,System.IO.FileInfo)")]
-[assembly: SuppressMessage("Major Code Smell", "S4457:Parameter validation in \"async\"/\"await\" methods should be wrapped", Justification = "<Pending>", Scope = "member", Target = "~M:MaxMind.Db.ArrayBuffer.BytesFromStreamAsync(System.IO.Stream)~System.Threading.Tasks.Task{System.Byte[]}")]
 [assembly: SuppressMessage("Style", "IDE0056:Use index operator", Justification = "<Pending>", Scope = "member", Target = "~M:MaxMind.Db.Buffer.ReadBigInteger(System.Int64,System.Int32)~System.Numerics.BigInteger")]
 [assembly: SuppressMessage("Style", "IDE0056:Use index operator", Justification = "<Pending>", Scope = "member", Target = "~M:MaxMind.Db.Reader.FindAll``1(MaxMind.Db.InjectableValues,System.Int32)~System.Collections.Generic.IEnumerable{MaxMind.Db.Reader.ReaderIteratorNode{``0}}")]