Nix Binary Cache

# Nix Binary Cache

An official public binary cache is not active yet. Nix installations may still
build Spotifast from source. The GitHub Actions cache speeds up CI only; it is
not a public Nix substituter.

## Maintainer setup

1. Create a public cache in a maintainer-owned
   [Cachix account](https://app.cachix.org/). Use Cachix-managed signing and
   create a token with permission to push to that cache.
2. Add the token as the GitHub repository secret `CACHIX_AUTH_TOKEN`. Do not
   put it in an issue, configuration file, or public documentation.
3. Set the repository variable `CACHIX_CACHE` to the cache's name. Leaving
   this variable unset keeps the existing CI build and cache behavior.
4. Push a checked change to `main`. After `nix build .#default` succeeds,
   the Nix CI job publishes the result and its dependency closure. A configured
   cache without its upload token fails publication instead of claiming success.
5. Verify the uploaded package can be substituted on a separate Nix machine.
   Then replace the pending notice here, and add to the Download page, the actual
   cache URL, public signing key, and user setup instructions from Cachix.

Pull requests and manual workflow runs only read the public cache. The upload
token is supplied only to the publication step on a push to `main`.

The current Nix job builds `x86_64-linux`. It does not populate macOS or ARM64
packages. A different flake revision, system, or build override can still need
a local build even after the cache is active. This cache does not publish a
GitHub application release or change its download artifacts.

See the [Cachix action documentation](https://github.com/cachix/cachix-action)
for how the cache is added as a substituter. The workflow uses the action only
for read access and explicitly pushes the successful package closure afterward.

An official public binary cache is not active yet. Nix installations may still build Spotifast from source. The GitHub Actions cache speeds up CI only; it is not a public Nix substituter.

Maintainer setup

Create a public cache in a maintainer-owned Cachix account. Use Cachix-managed signing and create a token with permission to push to that cache.
Add the token as the GitHub repository secret CACHIX_AUTH_TOKEN. Do not put it in an issue, configuration file, or public documentation.
Set the repository variable CACHIX_CACHE to the cache’s name. Leaving this variable unset keeps the existing CI build and cache behavior.
Push a checked change to main. After nix build .#default succeeds, the Nix CI job publishes the result and its dependency closure. A configured cache without its upload token fails publication instead of claiming success.
Verify the uploaded package can be substituted on a separate Nix machine. Then replace the pending notice here, and add to the Download page, the actual cache URL, public signing key, and user setup instructions from Cachix.

Pull requests and manual workflow runs only read the public cache. The upload token is supplied only to the publication step on a push to main.

The current Nix job builds x86_64-linux. It does not populate macOS or ARM64 packages. A different flake revision, system, or build override can still need a local build even after the cache is active. This cache does not publish a GitHub application release or change its download artifacts.

See the Cachix action documentation for how the cache is added as a substituter. The workflow uses the action only for read access and explicitly pushes the successful package closure afterward.