fix: replace silent back-reference skip with explicit reference-equality enforcement in ObjectPropertiesEqualsConstraint

[Copilot]

When comparing object graphs containing shared or circular references, the constraint silently skipped (continue) any property whose expected value had been seen before. This meant structural identity — e.g. two properties on the expected object pointing to the same instance — was never verified against the actual graph.

Changes

ComparisonContext (new)

• Construction-time registry mapping expected object identity → ref-id (via ReferenceComparer)
• Match-time binding of ref-id → actual object
• EnterMatch/ExitMatch depth tracking to reset bindings between ApplyTo calls

BackReferenceConstraint (new, inner class)

• Replaces the continue skip when a previously-registered expected object is encountered again
• Enforces ReferenceEquals(actual, previouslyBoundActual) — the same expected reference must correspond to the same actual reference

ObjectPropertiesEqualsConstraint

• InitializeProperties: on back-reference, creates BackReferenceConstraint instead of skipping; IEnumerable types are still delegated to EnumerableEqualsConstraint (no ref-tracking needed)
• Matches: calls EnterMatch/ExitMatch around traversal; binds each forward-reference node's actual value via BindActual
• ComparisonContext flows through nested constraints (replacing the old HashSet<object>)

ReferenceComparer

• Fixed GetHashCode to use RuntimeHelpers.GetHashCode (identity hash) instead of obj.GetHashCode(), which could be overridden

Tests

// Same expected instance referenced from two properties — actual uses different (but equal) instances → must fail
var sharedAddress = new Address { Street = "Main St" };
var expected = new PersonWithTwoAddresses { Home = sharedAddress, Work = sharedAddress };
var actual   = new PersonWithTwoAddresses { Home = new Address { Street = "Main St" },
                                            Work = new Address { Street = "Main St" } };
Assert.That(actual, Does.Not.Match(expected).Properties());

• Shared-ref with same actual instance → passes
• Two independent equal instances (not back-references) → passes
• Self-referencing expected vs. non-cyclic actual → fails