Make cs2gs produce runnable mirrored repositories

[DavidObando]

Summary

• make repository mirroring the default cs2gs migrate output, preserving directories and non-C# files while converting C# sources, projects, and solutions
• upgrade migrated NBGV references and generate G# project/solution metadata with correct dependency and friend-assembly handling
• fix compiler and translator gaps exposed by the Oahu migration, including attributes, nullability, object initializers, imported members, EF entities, and embedded resources
• retain the diagnostic corpus layout behind --diagnostic-run

Validation

• GSharp SDK tests: 44 passed
• GSharp solution Release build: succeeded with 0 warnings and 0 errors
• migrated Oahu tests: 349 passed
• migrated Oahu.App.gsproj: builds and launches successfully with the full library UI

Notes

Broader full-suite runs also surfaced unrelated/baseline-sensitive failures outside the Oahu acceptance path; the focused regressions and acceptance checks above pass.