Create cross-language parity and compatibility tests and run them in CI

## Objective

Build a parity suite covering VB-framework baselines and shared-model behavior from both VB and C#. Include event ordering, cancellation, exceptions, command lines, single-instance activation, splash timing, settings, network events, multi-user/session boundaries, and compatibility. Run locally and in CI.

## 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 reviewing the existing WinForms test and CI structure, then identify how VB-framework baselines and shared-model behavior can be exercised from both VB and C#. Done means the requested parity areas are covered, tests run locally and in CI, and important decisions, dependencies, alternatives, and follow-up work are documented.

Written by the indexing model from the issue text.