Run the simulation module unit suite in isolation

The separate test/simulation Go module is invisible to root ./... test discovery. This change makes its unit tests hermetic, fixes test fixture assumptions, and adds a repeatable simulation-module runner.

Validation: The agent reported 14 packages and 461 tests passing, including five shuffled isolation runs. This topic is also included in draft !5181 (closed). This MR will run its own CI against develop; review and merge coordination should avoid landing the same change twice.