ApiSutra PHP 0.1.2

• Expanded the runnable Records SDK with six related DTOs, typed collections, enum and
  discriminator variants, a bidirectional cast, inline Base64 and nested extra fields.
• Demonstrated recursive serialization, immutable copies, standalone hydration,
  missing/null/default rules and twelve invalid responses with field-level diagnostics.
• Updated the English and Russian walkthroughs and installation checks. No core runtime API changes.