[FEATURE] Track simulation time and expose as a signal

**Is your feature request related to a problem? Please describe.**
For timer-related stubs, it'd be useful to have a signal that the sim core updates.

**Describe the solution you'd like**
TODO

The issue names no files, tests, or entry points. Start by locating the simulation core and timer-related stubs, then determine how simulation time advances and how existing signals are exposed. Done means the core updates a defined simulation-time signal that timer-related stubs can use, with behavior covered by appropriate tests.

Written by the indexing model from the issue text.