Add native Wayland shell host scaffolding

Introduce a Linux-only C++20 `nebula-shell` host built with CMake, GTK4, gtk4-layer-shell, and WebKitGTK 6.0. Add a shared `Surface` abstraction and concrete Desktop/TopBar/Launcher surfaces with layer placement, sizing, transparency, and per-surface `?surface=` URL loading.

Add `ShellApplication` startup flow that validates `NEBULA_SHELL_DEV_URL`, checks layer-shell support, creates/presents desktop and top bar, and keeps launcher created but hidden for future bridge-driven toggling. Update the native README to document architecture, development setup, dependencies, and current milestone scope.

shells/desktop/native/CMakeLists.txt

@@ -0,0 +1,32 @@
+cmake_minimum_required(VERSION 3.20)
+
+# Linux-only. GTK4 Layer Shell and WebKitGTK are not built or stubbed on Windows.
+project(nebula-shell LANGUAGES CXX)
+
+set(CMAKE_CXX_STANDARD 20)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_EXTENSIONS OFF)
+
+find_package(PkgConfig REQUIRED)
+pkg_check_modules(GTK4 REQUIRED IMPORTED_TARGET gtk4)
+pkg_check_modules(GTK4_LAYER_SHELL REQUIRED IMPORTED_TARGET gtk4-layer-shell-0)
+pkg_check_modules(WEBKITGTK REQUIRED IMPORTED_TARGET webkitgtk-6.0)
+
+add_executable(nebula-shell
+    src/main.cpp
+    src/ShellApplication.cpp
+    src/Surface.cpp
+    src/DesktopSurface.cpp
+    src/TopBarSurface.cpp
+    src/LauncherSurface.cpp
+)
+
+target_link_libraries(nebula-shell PRIVATE
+    PkgConfig::GTK4
+    PkgConfig::GTK4_LAYER_SHELL
+    PkgConfig::WEBKITGTK
+)
+
+if(CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang")
+    target_compile_options(nebula-shell PRIVATE -Wall -Wextra)
+endif()