Fix dispatcher crash/deadlock, instruction analyzer, cache equality, task leak, and redirection protection

- MainThreadDispatcher: guard DispatchHook with try/catch so exceptions never escape to native caller; drain and fault pending work on Dispose; synchronize Execute/ExecuteAsync/Dispose against race/dispose.
- InstructionAnalyzer: require ModRM 0xEC for 0x83/0x81 sub-esp/rsp forms, rejecting unsafe RIP-relative or memory forms.
- PatternScannerCache: implement value equality on CacheKey so repeated scans actually hit cache.
- BackgroundTaskExecutor: add remote allocations to the free list immediately after VirtualAllocEx, before any write that could fail and leak.
- redirect: capture and restore original page protection in Apply/Remove instead of leaving target RWX.
- Regression tests for all six fixes.

Tests: 198 passing, 4 integration/interactive skipped.

WhiteMagicTest/Discovery/PatternScannerCacheTests.cs

@@ -1,3 +1,5 @@
+using System.Linq;
+using System.Reflection;
 using System.Runtime.InteropServices;
 using WhiteMagic;
 using WhiteMagic.Discovery;
@@ -43,6 +45,48 @@ public class PatternScannerCacheTests
 
             Assert.Equal(addr + 30, first);
             Assert.Equal(first, second);
+
+            // Value equality must mean the second call reused the cached entry.
+            var cacheField = typeof(PatternScannerCache).GetField("_cache", BindingFlags.NonPublic | BindingFlags.Instance)!;
+            var cacheDict = cacheField.GetValue(cache)!;
+            int count = (int)cacheDict.GetType().GetProperty("Count")!.GetValue(cacheDict)!;
+            Assert.Equal(1, count);
+        }
+        finally
+        {
+            pin.Free();
+        }
+    }
+
+    [Fact]
+    public void FindCached_value_equality_uses_content_not_reference()
+    {
+        using var reader = CreateReader();
+        var cache = new PatternScannerCache(reader);
+
+        byte[] buffer = new byte[256];
+        buffer[10] = 0xAA;
+        buffer[11] = 0xBB;
+
+        GCHandle pin = GCHandle.Alloc(buffer, GCHandleType.Pinned);
+        try
+        {
+            IntPtr addr = pin.AddrOfPinnedObject();
+            IntPtr end = addr + buffer.Length;
+
+            byte[] pattern1 = { 0xAA, 0xBB };
+            byte[] pattern2 = { 0xAA, 0xBB };
+
+            IntPtr first = cache.FindCached(pattern1, null, addr, end);
+            IntPtr second = cache.FindCached(pattern2, null, addr, end);
+
+            Assert.Equal(addr + 10, first);
+            Assert.Equal(first, second);
+
+            var cacheField = typeof(PatternScannerCache).GetField("_cache", BindingFlags.NonPublic | BindingFlags.Instance)!;
+            var cacheDict = cacheField.GetValue(cache)!;
+            int count = (int)cacheDict.GetType().GetProperty("Count")!.GetValue(cacheDict)!;
+            Assert.Equal(1, count);
         }
         finally
         {