Refactor code generation boundaries

[turinglambdaai]

Summary

• split schema loading, compatibility snapshots, and type-graph analysis out of the codegen facade
• clarify that generated native hosts use native system APIs directly instead of installing Racket procedures across RVT1
• make source-package smoke coverage require the new internal codegen dependency closure

Validation

• raco test tests/ (8663 tests)
• focused codegen tests (151 tests)
• Scribble HTML build
• source archive install and generated Swift/C++/Kotlin client smoke
• bash syntax validation
• git diff validation