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/env.nix

@@ -14,6 +14,15 @@
     # R.nvim v1.x owns its cache and temporary directories and exports
     # RNVIM_COMPLDIR/RNVIM_TMPDIR during setup. Do not inject literal `$PWD`
     # values into the wrapper environment.
+    #
+    # ponytail: with R_LIBS_USER unset-or-empty, loading compiled namespaces
+    # (S7 via btw) segfaults on macOS for bare-terminal launches outside the
+    # devShell hook. Any non-empty value avoids it; this store path exists,
+    # contains no libraries, and stays inert. The devShell hook overrides it
+    # with $PWD/.r-libs. Upgrade path: teach wlib to emit expandable defaults.
+    (lib.mkIf (config.cats.r or false) {
+      R_LIBS_USER = "${pkgs.rpkgs.rWrapper}";
+    })
   ];
 
   # Environment variables with defaults (can be overridden by user)