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.

WhiteMagicTest/Memory/AllocatedMemoryTests.cs

@@ -251,4 +251,73 @@ public class AllocatedMemoryTests
 
         Assert.Contains("does not exist", ex.Message);
     }
+
+    [Fact]
+    public void Read_T_throws_when_value_exceeds_allocation()
+    {
+        using var reader = CreateReader();
+
+        using var allocated = new AllocatedMemory(reader, 100);
+        allocated.AddRegion("boundary", 99);
+
+        var ex = Assert.Throws<ArgumentOutOfRangeException>(() =>
+            allocated.Read<int>("boundary"));
+
+        Assert.Equal("name", ex.ParamName);
+    }
+
+    [Fact]
+    public void Write_T_throws_when_value_exceeds_allocation()
+    {
+        using var reader = CreateReader();
+
+        using var allocated = new AllocatedMemory(reader, 100);
+        allocated.AddRegion("boundary", 99);
+
+        var ex = Assert.Throws<ArgumentOutOfRangeException>(() =>
+            allocated.Write("boundary", 42));
+
+        Assert.Equal("name", ex.ParamName);
+    }
+
+    [Fact]
+    public void ReadBytes_throws_when_count_exceeds_allocation()
+    {
+        using var reader = CreateReader();
+
+        using var allocated = new AllocatedMemory(reader, 100);
+        allocated.AddRegion("boundary", 99);
+
+        var ex = Assert.Throws<ArgumentOutOfRangeException>(() =>
+            allocated.ReadBytes("boundary", 2));
+
+        Assert.Equal("count", ex.ParamName);
+    }
+
+    [Fact]
+    public void WriteBytes_throws_when_span_exceeds_allocation()
+    {
+        using var reader = CreateReader();
+
+        using var allocated = new AllocatedMemory(reader, 100);
+        allocated.AddRegion("boundary", 99);
+
+        var ex = Assert.Throws<ArgumentOutOfRangeException>(() =>
+            allocated.WriteBytes("boundary", new byte[2]));
+
+        Assert.Equal("bytes", ex.ParamName);
+    }
+
+    [Fact]
+    public void Read_and_Write_at_exact_allocation_boundary_succeed()
+    {
+        using var reader = CreateReader();
+
+        using var allocated = new AllocatedMemory(reader, 100);
+        allocated.AddRegion("boundary", 96);
+
+        const int expected = unchecked((int)0xDEADBEEF);
+        Assert.True(allocated.Write("boundary", expected));
+        Assert.Equal(expected, allocated.Read<int>("boundary"));
+    }
 }