Fast-path the ASCII key digest with a big-endian load

src/DryDB/IKeyEncoding.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Buffers.Binary;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
@@ -262,11 +263,21 @@
     /// byte-lexicographic order. Keys sharing an 8-byte prefix collide and fall back
     /// to the full comparison.
     /// </summary>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public ulong GetKeyDigest(ReadOnlySpan<byte> key)
     {
-        var length = Math.Min(key.Length, sizeof(ulong));
+        if (key.Length >= sizeof(ulong))
+        {
+            // Read the first 8 bytes as a big-endian number: one unaligned load plus
+            // one byte-swap instruction (ldr + rev / mov + bswap).
+            return BinaryPrimitives.ReverseEndianness(
+                Unsafe.ReadUnaligned<ulong>(ref MemoryMarshal.GetReference(key)));
+        }
+
+        // Shorter keys only: a whole-ulong load would read past the key, so pack the
+        // bytes individually (zero padding the low bits).
         var digest = 0UL;
-        for (var i = 0; i < length; i++)
+        for (var i = 0; i < key.Length; i++)
         {
             digest |= (ulong)key[i] << (56 - i * 8);
         }
@@ -292,7 +303,7 @@
                 bytesWritten = Encoding.ASCII.GetBytes(keyString, destination);
                 return true;
             }
             catch (ArgumentException ex)
             {
                 bytesWritten = 0;
                 return false;
@@ -314,7 +325,7 @@
             bytesWritten = Encoding.ASCII.GetBytes(formattedString, destination);
             return true;
         }
         catch (ArgumentException ex)
         {
             bytesWritten = 0;
             return false;