Expand TurtleBot3 demo to multi-robot fleet

[bburda]

Summary

Transform the single TurtleBot3 demo into a proper 3-robot fleet demonstration showcasing ros2_medkit's multi-robot discovery capabilities and SOVD entity hierarchy.

---

Proposed solution

Current State

• Single TurtleBot3 in root namespace (/)
• Runtime-only discovery (no manifest)
• All nodes lumped together in single area

Target State

Fleet Demo
├── /robot1 (TurtleBot3 Burger)
│   ├── perception/
│   └── navigation/
├── /robot2 (TurtleBot3 Burger)
│   ├── perception/
│   └── navigation/
├── /robot3 (TurtleBot3 Waffle)
│   ├── perception/
│   └── navigation/
└── /fleet (Fleet Manager)

Additional context

Resources:

• ros2_medkit multi_robot_manifest.yaml example
• TurtleBot3 multi-robot tutorial: https://emanual.robotis.com/docs/en/platform/turtlebot3/simulation/#2-multi-robot

May need to simplify worlds for CI or add a param to run only a single TurtleBot