[Feature]: Implement `Resource.Save(...)` serialization back to `.ecore`

[samatstariongroup]

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of the EcoreNetto
• I have searched open and closed issues to ensure it has not already been reported

Description

• Summary: Resource.Save(...) logs a warning and throws NotImplementedException. There is no way to write a model back to .ecore, which limits round-tripping / model-authoring scenarios.
• Affected files: ECoreNetto/Resource/Resource.cs:289-294
• Proposed solution: Implement XML serialization that mirrors the deserialization performed in ECoreParser/ReadXml. Round-trip tests (load → save → reload → compare) make a strong acceptance signal.
• Acceptance criteria:
  • A loaded model can be saved and re-loaded with an equivalent object graph.
  • Round-trip tests pass for the sample models.