nix copy --from <attic> only pulls from that single store, so nixpkgs
deps (e.g. sqlite) that Attic skips and leaves on cache.nixos.org were
missing -> 'path is not valid'. Realize the closure through the machine's
substituters so the app comes from the dev Attic and its upstream deps
resolve from cache.nixos.org, without building from source.
Add a radiantwave-updater NixOS module that polls a manifest on the
fileserver and swaps the app pointer /var/lib/radiantwave/current to new
closures fetched from the dev Attic cache. Closures are fetched with
'nix copy --from', never built from source; the swap is atomic via
mv -T.
Enable it on the -dev variants only for now: the login shell launches
from /var/lib/radiantwave/current/bin/radiantwave when enabled, and from
the immutable system path otherwise, so prod behavior is unchanged. The
pointer is seeded on first boot to the baseline copy already baked into
the closure (no duplication).
NeilClack