Make settings.lang_packages append instead of replace

Downstream modules assign settings.lang_packages.<lang> plainly, which
out-priorities mkOptionDefault and silently dropped the built-in
defaults (e.g. lintr, breaking R linting in downstream wrappers like the
project-template ed flake).

Move the built-in defaults into a separate settings.langPackageDefaults
option and compose defaults ++ lang_packages at the consumption sites
(catPkgs r/python/julia, quarto extraRPackages, neovide R host). A new
append-semantics check fails if defaults migrate back into lang_packages.

Also fix pre-existing inverted assertions in downstream-overrides check:
the override module sets cats.r = true, so assert presence, not absence.

modules/module/settings/hosts.nix

@@ -5,7 +5,10 @@
   ...
 }:
 let
-  rPackages = (pkgs.baseRPackages or [ ]) ++ config.settings.lang_packages.r;
+  rPackages =
+    (pkgs.baseRPackages or [ ])
+    ++ config.settings.langPackageDefaults.r
+    ++ config.settings.lang_packages.r;
   rWrapperPkg = pkgs.rpkgs.rWrapper.override { packages = rPackages ++ [pkgs.nvimcom]; };
 in
 {