attic-closure archetype: no parity-lib builder exists for attic pushes, so
wrap the existing per-arch package build in ci/publish.py (woodpecker-peek
pattern) and expose `nix run .#{stage,publish}-amd64` + `.#publish`.
Two-halves rule: STAGE nix-builds every package in the arch list into the
local store (emmett-buildable); PUBLISH additionally attic-pushes each
closure. Local runs DRY-RUN unless --push/PUBLISH=1; CI sets PUBLISH=1.
The .woodpecker/{amd64,arm64}.yaml now call the same ci/publish.py so CI
and local runs can't drift. arm64 stays node-bound (no emmett cross path),
so it has no local-parity app. ci/build.py becomes a forwarding shim.
Oleks