Splits the platform-agnostic network core out of dusk into a top-level
dusknetwork module, mirroring the duskgl/dusksdl2 pattern. Adds a
DUSK_NETWORK cmake option (default ON) so builds can exclude all
networking code, including the per-platform implementations.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
