Add Brownian particle diffusion example (continuous space)

[yoginlangalia]

What this adds

A new example in the continuous space category — currently this section has no examples at all. The model simulates particles undergoing Brownian (random walk) motion in a torus-wrapped 2D space, with soft repulsion so they don't pile on top of each other.

Why I built this

I'm exploring the codebase and noticed the continuous space category was completely empty (the README literally says "No user examples available yet"). Seemed like a useful gap to fill — also a minimal reference for anyone wanting to use mesa.experimental.continuous_space without reading through a more complex example.

Files added

• examples/brownian_particles/brownian_particles/agents.py — Particle agent with random walk + soft repulsion
• examples/brownian_particles/brownian_particles/model.py — BrownianModel + BrownianScenario
• examples/brownian_particles/app.py — SolaraViz visualization, plasma colormap based on neighbor count
• examples/brownian_particles/README.md — description, how to run, parameters

How to run

cd examples/brownian_particles
solara run app.py

What I tested

Ran 10 steps with 20 and 80 particles, datacollector works, no errors on Python 3.13 + Mesa 3.5.1 + Windows.

Also updated README.md to replace the placeholder in the Continuous Space section.

Happy to adjust folder structure or naming if needed!