[Add] XmiWriter implementation; fixes #194

[samatstariongroup]

Summary

Implements #194: an XMI writer for uml4net.xmi that serializes a UML model (or a selected Package) back to XMI, mirroring the existing XMI reader architecture exactly.

• XmiWriterBuilder / XmiWriterScope / XmiWriter follow the same Autofac-based pattern as XmiReaderBuilder / XmiReaderScope / XmiReader.
• Sync and async Write APIs, writing to a file path or a Stream.
• External reference handling is selectable via IXmiWriterSettings.ExternalReferenceResolution:
  • Href (default) — references outside the selected package are written as href links to the original document.
  • Include — the containing root packages of external references are pulled into the output, producing a self-contained document.
• ReferenceClosureCalculator computes the containment/reference closure at runtime from the same [Property]/[SubsettedProperty] metadata used by the reader, including replicating the composite-serialized-as-reference case (e.g. Behavior.postcondition).
• 193 generated element writers + XmiElementWriterFacade in uml4net.xmi/AutoGenXmiWriters/, generated from new Handlebars templates/helpers in uml4net.CodeGenerator / uml4net.HandleBars, regenerable via the [Explicit] test XmiWriterGeneratorTestFixture.Regenerate_AutoGenXmiWriters_of_uml4net_xmi.
• README and CLAUDE.md updated to document the writer.

Testing

• Full solution test suite green (431 tests) including new round-trip tests (UML.xmi, SysML.uml) proving read → write → re-read produces an equivalent object graph, sync/async byte-identical output, and Include-mode self-containment.
• New-code coverage on hand-written writer classes is ≥94% (gate is 80%).

[sonarqubecloud]

Quality Gate failed

Failed conditions
4.8% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud