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/LauncherSurface.cpp

@@ -0,0 +1,21 @@
+#include "LauncherSurface.hpp"
+
+LauncherSurface::LauncherSurface(GtkApplication* app, const std::string& base_url)
+    : Surface(app, base_url, Config{
+          .layer_namespace = "nebula-launcher",
+          .surface_name = "launcher",
+          .layer = GTK_LAYER_SHELL_LAYER_OVERLAY,
+          .anchor_top = true,
+          .anchor_bottom = true,
+          .anchor_left = true,
+          .anchor_right = true,
+          .height_px = -1,
+          .keyboard_mode = GTK_LAYER_SHELL_KEYBOARD_MODE_ON_DEMAND,
+          .auto_exclusive_zone = false,
+      })
+{
+    // Created and fully configured, but left unmapped on purpose.
+    // Later, when shellBridge is wired to WebKitGTK, call present() to show
+    // the launcher and gtk_widget_set_visible(GTK_WIDGET(window()), FALSE) to hide it.
+    gtk_widget_set_visible(GTK_WIDGET(window()), FALSE);
+}