Implement hybrid package management: stable + selective unstable

- Add nixpkgs-unstable input to flake.nix
- Configure specialArgs to pass pkgs-unstable to all system configs
- Update desktop/dev.nix to use windsurf from unstable packages
- Create comprehensive documentation in README-hybrid-packages.md
- Enable selective use of cutting-edge packages while maintaining system stability

Usage: Add pkgs-unstable parameter to any .nix file and use pkgs-unstable.package-name
Example: pkgs-unstable.windsurf for latest Windsurf editor

desktop/dev.nix

@@ -1,4 +1,4 @@
-{ config, pkgs, ... }:
+{ config, pkgs, pkgs-unstable, ... }:
 
 {
 
@@ -6,6 +6,7 @@ environment.systemPackages = with pkgs; [
    (python3.withPackages(ps: with ps; [ pandas requests python-dotenv pip uv ]))
   nodejs
   rpi-imager
+  pkgs-unstable.windsurf  # Use windsurf from unstable packages
   ];
 
 programs.nix-ld.enable = true;