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/Execution/MainThreadPump.cs

@@ -83,7 +83,7 @@ public sealed class MainThreadPump : IDisposable
                 throw new InvalidOperationException("The main-thread pump is not installed. Call Install() first.");
 
             object? Box() => work()!;
-            _queue.Enqueue(new WorkItem(Box, r => tcs.SetResult((TResult)r!), ex => tcs.SetException(ex)));
+            _queue.Enqueue(new WorkItem(Box, r => tcs.TrySetResult((TResult)r!), ex => tcs.TrySetException(ex)));
         }
 
         return tcs.Task;
@@ -158,20 +158,34 @@ public sealed class MainThreadPump : IDisposable
         public WorkItem(Func<object?> work, TaskCompletionSource<object?> tcs)
         {
             Work = work;
-            _setResult = r => tcs.SetResult(r);
-            _setException = ex => tcs.SetException(ex);
+            _setResult = r => tcs.TrySetResult(r);
+            _setException = ex => tcs.TrySetException(ex);
         }
 
         public Func<object?> Work { get; }
 
         public void SetResult(object? result)
         {
-            _setResult?.Invoke(result);
+            try
+            {
+                _setResult?.Invoke(result);
+            }
+            catch (InvalidOperationException)
+            {
+                // Already completed, e.g. concurrent Dispose/PumpHook race.
+            }
         }
 
         public void SetException(Exception exception)
         {
-            _setException?.Invoke(exception);
+            try
+            {
+                _setException?.Invoke(exception);
+            }
+            catch (InvalidOperationException)
+            {
+                // Already completed, e.g. concurrent Dispose/PumpHook race.
+            }
         }
     }
 }