Add SEIR epidemic model with vaccination policy meta-agent

[BittuAgarwal1237]

Summary
Adds a SEIR epidemic model with a government vaccination policy meta-agent.
Motive
There's no population-level epidemic spreading example in mesa-examples. The existing virus_antibody model works at the biological/cellular level, but nothing shows how a disease spreads through a population on a grid and how a policy intervention affects that. This fills that gap.
Implementation
PersonAgents move through four states -Susceptible, Exposed, Infected, Recovered, based on contact with infected neighbors. A GovernmentAgent monitors the infection rate each step and triggers vaccination campaigns when it crosses a threshold. Activation is explicit and bottom-up: people act first, then the government responds. No legacy schedulers used.
Usage
pip install mesa[rec]
python -m solara run app.py
Closes #350
@tpike3 @EwoutH

[quaquel]

You opened many PRs, none with a clear motivation nor agreement that these changes are needed.

[BittuAgarwal1237]

Hi @quaquel, sorry for the confusion! PR #353 accidentally had both the hierarchical organization model and the SEIR model combined in the same branch, so I opened this as a clean PR with only the SEIR model. That's the only reason for the second PR
The original issue #350 was pre-approved before I opened the PR. Happy to go back to #353 if that's preferred, or follow whatever approach works best for the team