Add project structure and roadmap documentation

- Created `project-structure.md` to outline the directory layout, crate dependencies, design principles, module guidelines, and naming conventions for the NxMesh codebase.
- Introduced `roadmap.md` detailing the development phases, milestones, tasks, deliverables, and resource requirements for the NxMesh project, spanning from foundational setup to enterprise features.

.devcontainer/docker-compose.yml

@@ -27,7 +27,7 @@ services:
       - docker
     pid: "service:nginx"
 
-  # Data Plane - Nginx (controlled by agent via Docker)
+  # Data Plane - Nginx (controlled by agent via PID namespace sharing)
   nginx:
     image: nginx:alpine
     container_name: nxmesh-nginx