Fix bounds, memory protection, and completion race in core helpers

- AllocatedMemory.Read<T>/Write<T>/ReadBytes/WriteBytes now validate that
  the requested byte range stays within the allocated block before calling
  into the memory accessor.
- Patch.Apply/Remove temporarily changes the target page to read-write and
  restores the original protection, mirroring the Detour behavior.
- MainThreadPump.WorkItem uses TrySetResult/TrySetException and swallows the
  InvalidOperationException raised when a completion source is already
  completed, preventing Dispose from failing during concurrent pump drainage.

Regression tests added for all three fixes.

Tests: 206 passing, 4 skipped.

WhiteMagic/Hooking/Patch.cs

@@ -1,5 +1,7 @@
 using System;
 using System.Linq;
+using System.Runtime.InteropServices;
+using WhiteMagic.Native;
 
 namespace WhiteMagic.Hooking;
 
@@ -53,7 +55,31 @@ public sealed class Patch : IDisposable
             return;
 
         OriginalBytes = _memory.ReadBytes(Address, PatchBytes.Length);
-        _memory.WriteBytes(Address, PatchBytes);
+
+        if (!NativeMethods.VirtualProtectEx(
+            _memory.Handle,
+            Address,
+            PatchBytes.Length,
+            MemoryProtectionType.ExecuteReadWrite,
+            out MemoryProtectionType oldProtect))
+        {
+            int error = Marshal.GetLastPInvokeError();
+            throw new InvalidOperationException($"Failed to change target memory protection: error {error}");
+        }
+
+        try
+        {
+            _memory.WriteBytes(Address, PatchBytes);
+        }
+        finally
+        {
+            NativeMethods.VirtualProtectEx(
+                _memory.Handle,
+                Address,
+                PatchBytes.Length,
+                oldProtect,
+                out _);
+        }
     }
 
     /// <summary>Restores the original bytes if they were captured.</summary>
@@ -62,7 +88,31 @@ public sealed class Patch : IDisposable
         if (OriginalBytes is null)
             return;
 
-        _memory.WriteBytes(Address, OriginalBytes);
+        if (!NativeMethods.VirtualProtectEx(
+            _memory.Handle,
+            Address,
+            OriginalBytes.Length,
+            MemoryProtectionType.ExecuteReadWrite,
+            out MemoryProtectionType oldProtect))
+        {
+            int error = Marshal.GetLastPInvokeError();
+            throw new InvalidOperationException($"Failed to change target memory protection: error {error}");
+        }
+
+        try
+        {
+            _memory.WriteBytes(Address, OriginalBytes);
+        }
+        finally
+        {
+            NativeMethods.VirtualProtectEx(
+                _memory.Handle,
+                Address,
+                OriginalBytes.Length,
+                oldProtect,
+                out _);
+        }
+
         OriginalBytes = null;
     }