wip: feat: Just use straight.el...

Every Nixy solution I could find or conceive had too many points spread across
these various facets:
  - Flimsy (emacsWithPackagesFromUsePackage)
  - Inelegant (builtins.exec, manual listings)
  - Inflexible (emacsWithPackagesFromUsePackage)
  - Otherwise unergonomic (everything }:D)

Straight.el is sufficiently declarative, flexible, and Nix-friendly without
blindly bowing down to the imperialist NixOS user.  Now, Convenience and
ergonomics shan't be forsaken for the dogma that is total Nixation.

modules/home/impermanence.nix

@@ -18,6 +18,28 @@ in {
         default = [];
         type = types.listOf types.anything;
       };
+
+      cache = {
+        directories = mkOption {
+          description = ''
+            While functionally identical to `directories` (at the moment),
+            `cache.directories` carries additional semantics: these directories
+            /can/ be erased, but typically /shouldn't/ be.
+          '';
+          default = [];
+          type = types.listOf types.anything;
+        };
+
+        files = mkOption {
+          description = ''
+            While functionally identical to `files` (at the moment),
+            `cache.files` carries additional semantics: these files /can/ be
+            erased, but typically /shouldn't/ be.
+          '';
+          default = [];
+          type = types.listOf types.anything;
+        };
+      };
     };
   };
 
@@ -25,8 +47,8 @@ in {
     home.persistence = {
       "/persist/home/${config.home.username}" = {
         allowOther = true;
-        directories = cfg.directories;
-        files = cfg.files;
+        directories = cfg.directories ++ cfg.cache.directories;
+        files = cfg.files ++ cfg.cache.files;
       };
     };
   };