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/src/Surface.hpp

@@ -0,0 +1,40 @@
+#pragma once
+
+#include <gtk/gtk.h>
+#include <gtk4-layer-shell.h>
+
+#include <string>
+#include <string_view>
+
+// Shared Layer Shell + WebKitGTK surface. Subclasses only supply layout.
+class Surface {
+public:
+    struct Config {
+        const char* layer_namespace = nullptr;
+        const char* surface_name = nullptr;
+        GtkLayerShellLayer layer = GTK_LAYER_SHELL_LAYER_TOP;
+        bool anchor_top = false;
+        bool anchor_bottom = false;
+        bool anchor_left = false;
+        bool anchor_right = false;
+        int height_px = -1;
+        GtkLayerShellKeyboardMode keyboard_mode = GTK_LAYER_SHELL_KEYBOARD_MODE_NONE;
+        bool auto_exclusive_zone = false;
+    };
+
+    Surface(GtkApplication* app, const std::string& base_url, const Config& config);
+    ~Surface() = default;
+
+    Surface(const Surface&) = delete;
+    Surface& operator=(const Surface&) = delete;
+    Surface(Surface&&) = delete;
+    Surface& operator=(Surface&&) = delete;
+
+    void present();
+    GtkWindow* window() const { return window_; }
+
+private:
+    GtkWindow* window_ = nullptr;
+};
+
+std::string make_surface_url(std::string_view base_url, std::string_view surface_name);