feat(multi): tmux keys to toggle broadcast and exclude single panes

In windows opened by netssh multi, prefix S toggles broadcast for all
panes and prefix e excludes the current pane (a pane-level
synchronize-panes off, shown as [SOLO] in the border) or includes it
again. netssh sync --pane does the same from a shell.

S and e are unbound in a default tmux (E would be "spread evenly").
netssh writes the bindings to a small tmux config and sources it, binds
a key only if the user's tmux.conf does not use it (netssh's own earlier
binding is refreshed), and the bindings act only in windows marked
@netssh. multi.sync_key and multi.solo_key change the keys or turn them
off (none); also in the configurator. zellij syncs whole tabs only, so
excluding a pane is tmux-only.

testenv: the render test presses the keys through the xterm client
(send-keys -K) and checks [SOLO], that broadcast skips the excluded
pane, that S toggles, and that other windows are unaffected. The client
gets GNU grep: BusyBox grep -oF counted only one match per line.

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

testenv/tests/lib.sh

@@ -124,10 +124,10 @@ screen_text() {
   for i in "${!OCR_VARIANTS[@]}"; do ocr_variant "$1" "$i"; done
 }
 
-# ocr_norm maps characters OCR confuses in DejaVu Sans Mono (1/l/I/|, C/( , 0/O)
-# to one form; expected text gets the same mapping, so comparisons stay strict
-# about everything else.
-ocr_norm() { tr 'lI|(O' '111C0'; }
+# ocr_norm maps characters OCR confuses in DejaVu Sans Mono (1/l/I/|, C/( , 0/O,
+# typographic dashes for "-") to one form; expected text gets the same mapping,
+# so comparisons stay strict about everything else.
+ocr_norm() { sed 's/[‐‑‒–—―−]/-/g' | tr 'lI|(O' '111C0'; }
 
 # screen_shows NAME TEXT... — take a screenshot and check that OCR finds every TEXT.
 screen_shows() {