[FEATURE] Design execution timing mechanism rehaul

[yunlishao]

Is your feature request related to a problem?

As Real-Sim FIXS expands to support more XIL configurations and multi-rate co-simulation scenarios, the execution timing architecture needs a deliberate redesign. A formalized timing framework would enable real-time pacing, multi-rate execution, and predictable behavior under load.

Core areas to address:

• Execution timing contract: Establish a formal contract defining how simulation steps are scheduled, what timing guarantees are provided, and how components declare their timing requirements.
• Deadline miss behavior: Allow users to declare what should happen when a simulation step cannot meet its timing deadline (e.g., skip, catch up, warn, halt).
• Multi-rate co-simulation: Support components running at different rates (e.g., traffic at 10 Hz, vehicle dynamics at 100 Hz) with well-defined synchronization points.

Describe the solution you'd like

This is a design exploration issue. The goal is to investigate the three key ideas above and produce a design/implementation plan.

Scope of design work:

1. Explore what an execution timing contract should look like for FIXS — how should components declare their timing needs via config.yaml?
2. Explore user-declared deadline miss policies — what behaviors make sense, and how should they be configured?
3. Explore multi-rate support — how should TrafficLayer orchestrate components running at different frequencies, considering both SUMO (TrafficLayer-driven stepping) and VISSIM (DLL-driven stepping)?
4. Consider impact on existing XIL integrations (CarMaker, Simulink, Carla).
5. Identify sub-tasks to be filed as separate implementation issues under milestone 0.10.0.

Output:

• Design document with implementation plan (markdown in doc/ or wiki)
• List of follow-up implementation issues for milestone 0.10.0

Describe alternatives you've considered

• Keeping current execution model and handling timing externally per-component — fragments timing logic and makes system behavior harder to reason about.
• Incremental pacing additions without a broader timing contract — doesn't address multi-rate needs or deadline miss policies.

Additional context

Key files relevant to this design:

• TrafficLayer/TrafficLayer/mainTrafficLayer.cpp — main simulation loop
• CommonLib/TrafficHelper.h — SIM_STEP, runOneStepSimulation()
• CommonLib/TrafficHelper.cpp — step execution, SimulationMode handling
• CommonLib/ConfigHelper.h — configuration parsing

This is the first issue under the new milestone 0.10.0. Implementation issues will follow from the design output.