Add initial Wayland shell with clock and system tray

- Layer-shell bars drawn in software with tiny-skia and fontdue
- Clock widget centered at the top of the screen
- System tray in a top-right notch, acting as the
  StatusNotifierWatcher, with icons and right-click menus
- Live-reloaded TOML config for colors
  (~/.config/pell/config.toml)
- devenv setup for the Rust toolchain and Wayland libraries
- README: config, tray behaviour and AI disclosure

devenv.nix

@@ -0,0 +1,35 @@
+{
+    pkgs,
+    lib,
+    ...
+}:
+
+let
+    # Loaded at runtime via dlopen by wayland-client / xkbcommon / EGL crates.
+    runtimeLibs = with pkgs; [
+        wayland
+        libxkbcommon
+        libGL
+    ];
+in
+{
+    languages = {
+        rust = {
+            enable = true;
+            components = [ "rustc" "cargo" "clippy" "rustfmt" "rust-analyzer" ];
+        };
+    };
+
+    packages = with pkgs; [
+        marksman
+        pkg-config
+        wayland-protocols
+        wayland-scanner
+        fontconfig
+        freetype
+        # Linked by the `dbus` crate (system tray).
+        dbus
+    ] ++ runtimeLibs;
+
+    env.LD_LIBRARY_PATH = lib.makeLibraryPath runtimeLibs;
+}