Add discrete event simulation

[reuben-thomas]

Addition of Discrete Event Simulation (DES) Library

This PR introduces rmf_site_sim as a new crate for performing DES in Bevy, with a core API that should be intuitive to Bevy users.

Core Concepts and API

DiscreteEvent is very similar to a Bevy Command, and represents an instantaneous change in the state of the world.SimulationState is a wrapper around a Bevy World, representing the state vector at any given moment.
Prediction Systems are ordinary Bevy systems that are used to model behaviour. Rather than mutate the world directly, these systems propose CandidateDiscreteEvents.
CandidateDiscreteEvents are events that have been predicted by prediction systems that might occur. A DiscreteEvent within is only applied on the World if it is guaranteed to occur.

Computing a Simulation

The relationship between Prediction Systems CandidateDiscreteEvents, and DiscreteEvents in the computation of a simulation may be summarized in the following:

A simulation is computed asynchronously in a Bevy [AsyncComputeTaskPool], first by extracting all relevant entities, components, and resources from the main world, and sending SimulationStep updates per timestep to the Simulation in the main thread as they are computed.

Supporting Functionality

Synchronization

A simulation only ever handles a subset of components, entities, and resources, which is registered and managed by a Synchronizer, which is used to extract only required data for compute, as well as switching between simulations during playback.

Playback

The SimulationPlaybackPlugin plays a computed simulation
back on the main world with speed control, seeking, and replay behaviour.

Interaction

• keyboard A plugin for controlling playback with keyboard commands.
• egui (egui) Playback controls, an event table, and an overview panel implemented in egui.
• rmf_site_egui (rmf_site_egui) The above, as tiles in the RMF Site Editor's panels.

GenAI Use

• I used a GenAI tool in this PR.
• I did not use GenAI

Some portions of this pull request were generated using:

• Claude Code: Sonnet 4.6, Sonnet 5
• copilot.lua: GitHub Copilot Completions API
• Zed: Zed Completions API