tmux: mouse back on -- it was never the cause either

Measuring display columns instead of string length shows the status bar uses
74 of 158 available columns with the circled-number formats in place. The
"92 chars" that justified both the format change and turning the mouse off
was counting #[fg=...] style directives, which take no screen width at all.

So nothing here starves anything, and the bar still blanks in every mouse
state. Keeping mouse off bought nothing and cost scrolling.

The blanking is handled by tmux-statusline-keepalive.service instead: a full
refresh-client every 2s across each socket, 0.1% of a core. That is a
mitigation, not a fix -- the cause is still unknown.

.tmux.conf

@@ -69,7 +69,7 @@ bind E setw synchronize-panes off
 # set first window to index 1 (not 0) to map more to the keyboard layout...
 set-option -g base-index 1
 set-window-option -g pane-base-index 1
-set-window-option -g mouse off
+set-window-option -g mouse on
 
 # color scheme (styled as vim-powerline)
 set -g status-left-length 52