refactor: use pipe for clearer data transformations in overlay and home

Replace multiple intermediate let bindings with pipe for clearer data flow
in default-overlay creation and split-user-and-host function.

Changes:
- overlay/default.nix: Added pipe and flatten to imports
- overlay/default.nix: Refactored default-overlay using pipe (5 bindings → 1 pipeline)
- home/default.nix: Added pipe to imports
- home/default.nix: Refactored split-user-and-host using pipe

Impact:
- Lines: +8 -14 (net: -6)
- Eliminated 6 intermediate bindings total
- Clearer data transformation flow
- More maintainable code

Testing: nix flake check ✓

snowfall-lib/home/default.nix

@@ -26,6 +26,7 @@ let
     types
     hasInfix
     hasSuffix
+    pipe
     ;
 
   user-homes-root = snowfall-lib.fs.get-snowfall-file "homes";
@@ -64,9 +65,10 @@ in
     split-user-and-host =
       target:
       let
-        raw-name-parts = builtins.split "@" target;
-        name-parts = builtins.filter builtins.isString raw-name-parts;
-
+        name-parts = pipe target [
+          (builtins.split "@")
+          (builtins.filter builtins.isString)
+        ];
         user = builtins.elemAt name-parts 0;
         host = if builtins.length name-parts > 1 then builtins.elemAt name-parts 1 else "";
       in

snowfall-lib/overlay/default.nix

@@ -5,7 +5,7 @@
   snowfall-config,
 }:
 let
-  inherit (core-inputs.nixpkgs.lib) assertMsg foldl concatStringsSep;
+  inherit (core-inputs.nixpkgs.lib) assertMsg foldl concatStringsSep pipe flatten;
 
   user-overlays-root = snowfall-lib.fs.get-snowfall-file "overlays";
   user-packages-root = snowfall-lib.fs.get-snowfall-file "packages";
@@ -191,18 +191,12 @@ in
 
         default-overlay =
           final: prev:
-          let
-            overlays-list = builtins.attrValues overlays;
-            package-overlays-list = builtins.attrValues package-overlays;
-
-            overlays-results = builtins.map (overlay: overlay final prev) overlays-list;
-            package-overlays-results = builtins.map (overlay: overlay final prev) package-overlays-list;
-
-            merged-results = snowfall-lib.attrs.merge-shallow-packages (
-              package-overlays-results ++ overlays-results
-            );
-          in
-          merged-results;
+          pipe [ package-overlays overlays ] [
+            (builtins.map builtins.attrValues)
+            flatten
+            (builtins.map (overlay: overlay final prev))
+            snowfall-lib.attrs.merge-shallow-packages
+          ];
       in
       package-overlays // overlays // { default = default-overlay; } // extra-overlays;
   };