Move network code into its own dusknetwork module, gate behind DUSK_NETWORK

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>

src/duskpsp/CMakeLists.txt

@@ -18,7 +18,9 @@ target_sources(${DUSK_BINARY_TARGET_NAME}
 add_subdirectory(asset)
 add_subdirectory(input)
 add_subdirectory(log)
-add_subdirectory(network)
+if(DUSK_NETWORK)
+  add_subdirectory(network)
+endif()
 add_subdirectory(save)
 add_subdirectory(system)
 add_subdirectory(time)