Fix StackOverflow on circular object graphs by restoring ReferenceComparer

[Copilot]

When traversing object graphs with circular references, the HashSet<object> used to track visited nodes was inadvertently relying on object.Equals(). For types that override Equals() with property-walking logic, this re-enters the constraint traversal, causing a StackOverflow.

Changes

• ObjectPropertiesEqualsConstraint.cs: Restore new ReferenceComparer() as the HashSet comparer — uses ReferenceEquals (pointer comparison) instead of object.Equals(), guaranteeing cycle detection without recursion.

// Before (broken — triggers Equals() which can recurse)
private readonly HashSet<object> references = new HashSet<object>(/*new ReferenceComparer()*/);

// After (correct — pure pointer comparison, no recursion)
private readonly HashSet<object> references = new HashSet<object>(new ReferenceComparer());