Feature/readonly collections and adapters

[kasyanovandrii]

What's new

New packages

• Equatable.Generator.DataContract — adapter generator that reads [DataMember] attributes
  (System.Runtime.Serialization). Only properties annotated with [DataMember] are included in equality;
  [IgnoreDataMember] and unannotated properties are excluded. EQ0022 is emitted for any public property with no
  annotation, requiring the intent to be made explicit.
• Equatable.Generator.MessagePack — adapter generator that reads [Key(n)] attributes. Only [Key] properties are
  included; [IgnoreMember] and unannotated properties are excluded. EQ0023 is emitted for unannotated properties.

New features

• [DictionaryEquality(sequential: true)] — key-sorted dictionary comparison. Both sides are sorted by key before
  comparing, producing deterministic equality regardless of insertion order. Useful for snapshot testing and diagnostic
  logging. Propagates into nested dictionary values.
• Direction overrides — [HashSetEquality] on List or T[] produces order-insensitive comparison; [SequenceEquality]
  on HashSet enforces order-sensitive comparison.
• Nested collection comparer propagation — a single annotation on the outer property propagates the chosen comparer
  kind into all nested levels. Dictionary<K, Dictionary<K2,V>>, Dictionary<K, List>, and three-level nesting are all
  handled with a single annotation.
• MultiDimensionalArrayEqualityComparer — structural equality for T[,], T[,,], and higher-rank arrays, applied
  automatically as the default. Checks rank, dimension lengths, and element values in row-major order.
• IReadOnlyDictionary<K,V> support — dictionary comparers now accept any IReadOnlyDictionary<K,V>, not only
  Dictionary<K,V>.
• Base class delegation — the generated Equals method calls base.Equals() when the base class is also an
  equatable-generated type, including across adapter boundaries (e.g. [Equatable] derived from [DataContractEquatable]).
• Analyzer diagnostics
  • EQ0014 — equality attribute on a multi-dimensional array (rank ≥ 2), where the comparer cannot be overridden
  • EQ0015 — [SequenceEquality] or [HashSetEquality] on a dictionary type
  • EQ0020 — [DataContractEquatable] without [DataContract]
  • EQ0021 — [MessagePackEquatable] without [MessagePackObject]
  • EQ0022 — unannotated public property on a [DataContractEquatable] type
  • EQ0023 — unannotated public property on a [MessagePackEquatable] type

Bug fixes

• Empty collections hash differently from null — a sentinel value ensures GetHashCode(empty) != GetHashCode(null),
  satisfying the hash contract.
• HashSetEqualityComparer.GetHashCode is order-independent — uses a commutative accumulation strategy, consistent with
  SetEquals-based Equals.
• [DictionaryEquality(sequential: true)] propagates correctly — key-sorted mode is applied to nested dictionary
  values, not only the outermost level.
• Value types without == — EqualityComparer.Default is used instead of direct operator comparison.

Improvements

• Equatable.Generator.DataContract and Equatable.Generator.MessagePack are independent NuGet packages — include only
  what the project requires.
• IsPublicInstanceProperty extracted as a shared helper, eliminating duplication across adapter generators.
• Allocation-free hash code computation for dictionary comparers.

[kasyanovandrii]

@pwelter34, @loresoft — PR with adapter generators, nested collection propagation, multi-dimensional array support, and new analyzer diagnostics (EQ0014/0015/0022/0023). Would appreciate a review when you have time.