Design shared VB/C# application-feature contracts and migration adapters

## Objective

Map confirmed capabilities onto the shared builder/lifetime/services model. Define language-neutral contracts plus VB-friendly and C#-friendly usage, compatibility adapters, opt-in boundaries, event ordering, threading, cancellation, error semantics, and explicit non-goals. Do not plan removal of the existing VB framework.

## Completion guidance

- Keep the implementation focused on the objective while allowing the assignee to improve the approach based on research.
- Document important decisions, rejected alternatives, dependencies, and follow-up work.
- Add or update tests and documentation appropriate to the work.
- Verify locally and in CI where the task changes code or generated artifacts.

Start by researching the existing VB framework and the shared builder, lifetime, and services model described in the objective. Define the language-neutral contracts, VB- and C#-friendly usage, adapters, boundaries, ordering, threading, cancellation, and error semantics; document decisions and non-goals, then add appropriate tests and documentation and verify locally and in CI.

Written by the indexing model from the issue text.