[DarthPedro]

Added

• Fix zero-copy regression in span-based Deserialize
• Polymorphic / type-discriminator serialization
• Streaming (Stream + async) overloads
• Record / init-only property support audit (audited, gaps documented)
• Converter factories for open generics
• Field rename aliasing
• Source generator: support init-only property accessors
• Constructor-based construction for positional records
• Updated reflection and AOT samples to include round-trip serialization of records and polymorphic types.

Changed

• Split the oversized ModelBuilderDiagnosticTests test class in
  D20Tek.Serialization.Generator.Tests into two focused classes to improve maintainability:
  • ModelBuilderDiagnosticTests now contains only the general diagnostic and member-filtering
    tests (get-only/private setter, unsupported member type, nested type, static property,
    non-public property, indexer exclusion, and member declaration order).
  • ModelBuilderConstructorTests is a new test class containing all constructor-resolution
    coverage tests (unmatched constructor parameters, parameter type mismatches, multi-constructor
    matching, non-public/static constructor filtering, and static/indexer/non-public property
    name-collision cases).