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/dusknetwork/CMakeLists.txt

@@ -0,0 +1,13 @@
+# Copyright (c) 2026 Dominic Masters
+#
+# This software is released under the MIT License.
+# https://opensource.org/licenses/MIT
+
+# Includes
+target_include_directories(${DUSK_LIBRARY_TARGET_NAME}
+  PUBLIC
+    ${CMAKE_CURRENT_LIST_DIR}
+)
+
+# Subdirs
+add_subdirectory(network)