Function emit_flake_nix 

pub fn emit_flake_nix(spec: &EscribaPluginSpec) -> String

Render a minimal, real flake.nix for a standalone plugin caixa: a pure-source package that copies the caixa tree into $out so it can be fetched + materialized into escriba’s plugins dir. Follows the pleme-io flake-input rule (inputs.nixpkgs.follows-friendly); in the fleet, plugin caixas are consumed as flake = false source by the escribamourne distribution, so these per-plugin pins never compound into the closure.