Preview rendering

ekn render --package-placeholders and ekn diff --package-placeholders defer nixkube's node and cache closures. These commands use ekn.lib.packageRef before interpolating package paths. Normal commands retain real store paths and their string context.

Each role's internal nixConfig.nixConf resolves to { file, text }. The default generator uses nixpkgs' formatter for both representations. Preview reads text; normal rendering reads the generated file, including its build-time validation.

Existing custom package assignments remain valid:

nixkube.node.nixConfig.nixConf = lib.mkForce (pkgs.writeText "nix.conf" "max-jobs = 2\n");

The module converts such a package into { file = package; text = null; }. Both modes then read the custom file. A package's arbitrary .text attribute is ignored. To defer a custom file, supply its exact text explicitly:

let
  text = "max-jobs = 2\n";
in
{
  nixkube.node.nixConfig.nixConf = {
    file = pkgs.writeText "nix.conf" text;
    inherit text;
  };
}

A file-only override clears the default preview text. Keep explicit custom text equal to the file contents. The custom file controls its own validation.