feat: implement modular foundation and core system services

- Update flake.nix with 25.11 release and core inputs
- Add dedicated modules for audio (Pipewire), bluetooth, and networking
- Update GEMINI.md with the new Modular Merging Architecture blueprint
- Configure graphical installer ISO and test VM outputs

modules/system/network.nix

@@ -0,0 +1,5 @@
+{ config, pkgs, ... }:
+
+{
+  networking.networkmanager.enable = true;
+}