[tillig]

Summary

Documents the .NET AOT/trimming changes landing in core Autofac and the Aggregate Services extension.

• Adds a new Native AOT and Trimming advanced topic (docs/advanced/native-aot-trimming.rst) covering what works, what warns and why, what fails at runtime under Native AOT, and how to read/resolve IL2026/IL3050 warnings.
• Folds the legacy Xamarin / .NET Native guidance into that page and removes the old cross-platform-apps.rst, updating the advanced index and the Xamarin FAQ accordingly.
• Rewrites the Aggregate Services doc to explain the new source generator (the default, AOT/trim-safe path), the dynamic proxy runtime fallback, what the generator supports, and the AGSVC001 diagnostic.
• Adds AOT/trimming notes to the open-generic registration and assembly-scanning docs.

Notes

• This is a documentation-only change (Sphinx/RST).
• RST section underlines, internal cross-references, and the removed-file references were verified; no trailing-whitespace issues.