Part of kcenon/common_system#456

What

Change the CMake export namespace from NetworkSystem:: to network_system::.

• Current: NAMESPACE NetworkSystem::
• Expected: NAMESPACE network_system::
• Scope: cmake/NetworkSystemInstall.cmake, Config template

Why

Ecosystem convention is <name>:: with snake_case. Standardizing matches vcpkg port naming and find_package() package name. The Config template already provides NetworkSystem_FOUND compatibility variables.

How

1. Change NAMESPACE NetworkSystem:: to NAMESPACE network_system:: in NetworkSystemInstall.cmake
2. Update Config template target references
3. Add backward compatibility aliases if not already present

Acceptance Criteria

• Primary namespace is network_system::
• Backward compatibility aliases provided for NetworkSystem::
• Export file name remains network_system-targets.cmake (already correct)