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>

CMakeLists.txt

@@ -13,6 +13,7 @@ cmake_policy(SET CMP0079 NEW)
 # set(FETCHCONTENT_UPDATES_DISCONNECTED ON)
 
 option(DUSK_BUILD_TESTS "Enable tests" OFF)
+option(DUSK_NETWORK "Enable network support" ON)
 
 set(DUSK_GAME_NAME "Dusk" CACHE STRING "Game display name")
 set(DUSK_GAME_AUTHOR "YourWishes" CACHE STRING "Game author / coder")
@@ -90,6 +91,12 @@ target_compile_definitions(${DUSK_LIBRARY_TARGET_NAME}
     DUSK_VERSION="${DUSK_VERSION}"
 )
 
+if(DUSK_NETWORK)
+  target_compile_definitions(${DUSK_LIBRARY_TARGET_NAME} PUBLIC
+    DUSK_NETWORK
+  )
+endif()
+
 # Toolchains
 include(cmake/targets/${DUSK_TARGET_SYSTEM}.cmake)