perf: Optimize non-prefix key strategies and

memory usage

- Conditionally allocate prefix buffers in `LeafNode` and introduce `PrefixInternalNode` to reduce memory overhead when prefixes are disabled.
- Bypass prefix calculation and logic entirely when `UsesPrefixes` is false.
- Add a binary search fallback for key scanning.
- Implement a dedicated `int` scanning fast-path, removing SIMD prefix usage from `IntStrategy`.
- Reorganize key strategies into separate files.
- Add a new benchmark project specifically for string keys.

TestProject1/StandardStrategy.cs

@@ -0,0 +1,38 @@
+namespace PersistentMap.Tests;
+using PersistentMap;
+using System.Linq;
+using Xunit;
+public class StandardStrategy
+{
+    private static string GenerateRandomString(int length, Random rnd)
+    {
+        const string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
+        return new string(Enumerable.Repeat(chars, length).Select(s => s[rnd.Next(s.Length)]).ToArray());
+    }
+
+    [Fact]
+    public void Setup()
+    {
+        var N = 1000;
+        var _stdStrategy = new StandardStrategy<string>();
+        var _uniStrategy = new UnicodeStrategy();
+        var rnd = new Random(42);
+        var StringLength = 10;
+        // Build random strings
+        var _allKeys = Enumerable.Range(0, N).Select(_ => GenerateRandomString(StringLength, rnd)).Distinct().ToArray();
+        
+        // Regenerate if Distinct() reduced array size (highly unlikely with length 8/50, but safe)
+        while (_allKeys.Length < N)
+        {
+            _allKeys = _allKeys.Concat(new[] { GenerateRandomString(StringLength, rnd) }).Distinct().ToArray();
+        }
+
+        var transStd = BaseOrderedMap<string, int, StandardStrategy<string>>.CreateTransient(_stdStrategy);
+        var transUni = BaseOrderedMap<string, int, UnicodeStrategy>.CreateTransient(_uniStrategy);
+        for (int i = 0; i < _allKeys.Length; i++)
+        {
+            transStd.Set(_allKeys[i], i);
+            transUni.Set(_allKeys[i], i);
+        }
+    }
+}