Refactor: Split Assert.AreEquivalent.Comparer.cs into focused files

[Copilot]

Assert.AreEquivalent.Comparer.cs had grown to 1248 lines with four logically distinct responsibilities bundled together, making navigation and maintenance difficult.

Changes

Splits the file into focused partial class Assert files, moving private helper types to their own homes:

• Assert.AreEquivalent.Mismatch.cs (161 lines) — EquivalenceMismatch: result DTO with all failure-kind factory methods
• Assert.AreEquivalent.DictionaryView.cs (188 lines) — DictionaryView (abstract), NonGenericDictionaryView, GenericDictionaryView, GenericDictionaryAccessors, DictionaryLookup
• Assert.AreEquivalent.MemberAccessor.cs (70 lines) — MemberAccessor, MemberLookup, ReferenceObjectComparer
• Assert.AreEquivalent.Comparer.cs trimmed to 850 lines — EquivalenceComparer core orchestration only

All types remain private nested classes within partial class Assert. No logic changes, no public API impact.

The trimmed Comparer.cs exceeds the 300-line target because EquivalenceComparer is a single cohesive recursive algorithm that cannot be meaningfully decomposed further without splitting logic across files.