refactor: use __fish_palette in small utility functions (2/2)

25 duplicated declarations replaced by 7 calls. Completes the 33 structural
conversions: 167 declarations removed, 33 calls added, and --structural
confirms every one of the 33 diffs is declaration removals plus inserted
calls only -- no rendering text touched.

functions/y.fish is the one file in the repo with no trailing newline on
main; its original file ending is preserved rather than normalised.

functions/split.fish

@@ -31,8 +31,7 @@
 function split --description 'Run a command in a new terminal split'
     # Opinionated guard (C4): integrations disabled
     if not __fish_config_op_enabled (status current-function)
-        set -l c_err (set_color red)
-        set -l c_reset (set_color normal)
+        __fish_palette
         echo "$c_err"'split: disabled by __fish_config_op_integrations'"$c_reset" >&2
         return 1
     end