tmux: circled window numbers via an async hook, not a per-redraw conditional

The old format nested ten #{?#{==:#{window_index},N},...} conditionals inside
window-status-format, so tmux re-evaluated all ten for every window on every
redraw -- and redraws follow PANE ACTIVITY, not status-interval. A busy pane
ran them several times a second.

tmux-window-icons stamps each window with its symbol as a window-level user
option, so the format is a plain #{?#{@icon},#{@icon},#{window_index}} lookup.
It runs from run-shell -b on window-change hooks only, never on the render path.

Deliberately NOT a #() job: those expand EMPTY while restarting, which is the
bug that made status elements wink out in the first place. Reading a pre-set
option means a slow or failed run leaves the previous icon in place instead of
blanking the field.

Hooks are limited to those tmux 3.7b actually has -- after-kill-window,
after-move-window and after-swap-window do not exist and error on source.
window-unlinked covers kills; session-window-changed catches move/swap, which
have no hook of their own.

.local/bin/tmux-window-icons

@@ -0,0 +1,24 @@
+#!/bin/bash
+# Stamp every window with its circled-number icon as a WINDOW-level user option.
+#
+# Why not do this in the format string: the obvious approach nests ten
+# #{?#{==:#{window_index},N},...} conditionals, which tmux re-evaluates for
+# every window on every redraw -- and redraws are driven by PANE ACTIVITY, so a
+# busy pane re-runs all ten per window, several times a second.
+#
+# Here the work happens ONCE per window change (driven by hooks) and the render
+# path becomes #{@icon}: a plain variable lookup, no conditionals, no forks.
+#
+# Do NOT call this from the status format itself -- that would make it a #()
+# job, which renders EMPTY while restarting and is the original bug.
+symbols=(❶ ❷ ❸ ❹ ❺ ❻ ❼ ❽ ❾ ❿)
+while IFS= read -r w; do
+    idx=${w##*:}
+    [[ $idx =~ ^[0-9]+$ ]] || continue
+    if (( idx >= 1 && idx <= ${#symbols[@]} )); then
+        icon=${symbols[idx-1]}
+    else
+        icon=$idx
+    fi
+    tmux set -w -t "$w" @icon "$icon" 2>/dev/null
+done < <(tmux list-windows -a -F '#{session_name}:#{window_index}' 2>/dev/null)