#122 Add the per-timestep message flow diagrams

[YuSun-go]

Summary

Add documentation describing the traffic–vehicle co-simulation message flow used in the FIXS framework.

This PR introduces a new documentation page that explains the interaction between the following components:

• SUMO traffic simulation
• Traffic layer (C++ / FIXS)
• Python controller
• Simulink high-fidelity vehicle dynamics model

The document also includes several message flow diagrams that illustrate:

• The overall co-simulation architecture
• The current implementation message flow
• The baseline traffic-only model
• The final target co-simulation architecture

These diagrams aim to make the system architecture easier to understand for developers and new contributors.

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Related Issues / Tasks

PR 3 for #122

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Type of Change

• Bug fix
• New feature
• Maintenance / Refactor
• Documentation
• Test case / scenario update

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Affected Modules / Components

Documentation under:

doc/FIXS_message_flow.md
doc/images/

New diagrams added:

• FIXS_final_model.png
• Message_flow_1.png
• Message_flow_2.png
• Message_flow_3.png

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Test Cases

No runtime tests are required for this PR since it only introduces documentation and architecture diagrams.

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Environment

Not applicable (documentation update only).

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Checklist

• Code compiles/runs as expected
• Tests pass locally
• Documentation is updated (if applicable)
• Relevant issues are linked
• Version-specific changes are noted (if any)

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Additional Notes (optional)

The message flow diagrams are intended to clarify the data exchange between traffic simulation and high-fidelity vehicle dynamics in the co-simulation framework.

They also distinguish between:

• the baseline traffic simulation setup, where vehicle does not obtain a speed command from the controller, and
• the co-simulation framework, where vehicle commands are processed by a high-fidelity Simulink vehicle model before being fed back to the traffic simulator.

This documentation is meant to support future development and improve onboarding for contributors working on the co-simulation architecture.