Make the baked app a real store-path dependency via builtins.storePath

The previous commit wrapped the manifest's storePath string in a
symlinkJoin. symlinkJoin passes its paths as a file, so a bare store-path
string registered no dependency: Nix never substituted the closure and the
symlinkJoin failed with 'radiantwave-9b57e48: No such file or directory'.

builtins.storePath turns the published path into a real store-path
dependency that Nix substitutes from the channel's Attic cache, so the app
is baked and never compiled. Keep the raw string (publishedPath) for the
updater's plain-string baseline pointer.

configuration.nix

@@ -1,15 +1,10 @@
 { config, pkgs, radiantwaveApp, upgradeFlakeRef, radiantwavePlymouth, ... }:
 
 let
+  # The published app is a store path with context (builtins.storePath in
+  # flake.nix), so Nix substitutes it from the channel's Attic cache and the
+  # system closure depends on it.
   radiantApp = radiantwaveApp;
-  # The published app arrives as a bare store-path string (from the channel
-  # manifest, see flake.nix). Wrap it in a derivation built with this system's
-  # pkgs so it can live in environment.systemPackages; the wrapper is a symlink
-  # farm and the real closure is substituted from the channel's Attic cache.
-  radiantAppPackage = pkgs.symlinkJoin {
-    name = "radiantwave-published";
-    paths = [ radiantApp ];
-  };
   # When the live-updater module is enabled, RadiantWave launches through the
   # mutable /var/lib/radiantwave/current
   # pointer so it can be swapped without a full nixos-rebuild. Otherwise it
@@ -40,7 +35,7 @@ in
   # Included packages
   environment.systemPackages = [
     pkgs.cage pkgs.rsync pkgs.dbus pkgs.mesa pkgs.git pkgs.tailscale pkgs.sqlite pkgs.wireplumber
-    radiantAppPackage
+    radiantApp
   ];
 
   # This populates the EFI partition and tells the NUC how to start NixOS.