Support expression ["instanceContext", "taskID"] or similar

### Description

There is currently no trivial way for frontend components to know which task they are on in the process. This has ramifications for apps where multiple tasks use the same layout and/or data model.

### Additional Information

_No response_

The issue names no files, tests, or entry points. Start by locating how frontend components access process context and how expressions such as ["instanceContext", "taskID"] are evaluated. Done means a documented, tested way for components to identify their current task.

Written by the indexing model from the issue text.