sysvar: SELECTIONCYCLING

Selection cycling was reachable only from the status bar and Options.
SELECTIONCYCLING / SETVAR now read and set it: 0 off, 1 or 2 on (the
candidate list always shows).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/commands/mod.rs

@@ -757,6 +757,7 @@ inventory::submit!(crate::command::CommandRegistration {
         "FIELDDISPLAY",
         "FILEDIA",
         "TRIMEXTENDMODE",
+        "SELECTIONCYCLING",
         "SURFACEASSOCIATIVITY",
         "LAYOUTCREATEVIEWPORT",
         "FIELDEVAL",