feat: add COMMANDLINEFADETIME sysvar for overlay history fade

Controls how long command-line history lines stay visible, in ms (0-60000, default 3000). Bare form stays active until a value or Esc.

src/app/commands/mod.rs

@@ -534,6 +534,7 @@ inventory::submit!(crate::command::CommandRegistration {
         "SKPOLY",
         "SKTOLERANCE",
         "CLIPROMPTLINES",
+        "COMMANDLINEFADETIME",
         "COLORTHEME",
         "SELECTIONAREA",
         "SELECTIONAREAOPACITY",