dotcli: Add dot kde diff for schema-backed settings, fix apply/diff completions

Implements the broad, read-only schema-backed scan for dot kde diff:
walks every (rcfile, group, key) reachable through the kcfg mapping
table, compares live values against their schema-declared defaults,
and reports each mismatch tagged declared or undeclared. Never writes.

Also fixes dot kde completions, which only ever offered save/help —
apply (added in a prior task) and diff were both missing.

.config/dot/commands/kde/kde.fish

@@ -3,6 +3,7 @@ function _dot_kde_usage
 
 Commands:
   apply   push manifest entries onto the live system
+  diff    scan for settings whose live value differs from its default
   save    write live KDE settings into the manifest
   help    show this message
 
@@ -21,6 +22,9 @@ function _dot_kde
         case apply
             python3 $helper_dir/kde.py apply $argv[2..-1]
             return $status
+        case diff
+            python3 $helper_dir/kde.py diff $argv[2..-1]
+            return $status
         case save
             python3 $helper_dir/kde.py save $argv[2..-1]
             return $status