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.
