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/16-render.sh

@@ -32,13 +32,34 @@ visible "tmux: borders show host and [SYNC]" tmux-1 "$p1 [SYNC]" "$p2 [SYNC]" "$
 side_by_side tmux-1 "$p1" "$p2" && ok "tmux: tiled, first two panes side by side" || bad "tmux: tiled, first two panes side by side" "$(screen_text tmux-1)"
 tmux send-keys -t netssh-pve "echo typed-once-\$(hostname)" Enter
 visible "tmux: broadcast output in every pane" tmux-2 "typed-once-fsn1-pve1" "typed-once-fsn1-pve2" "typed-once-fsn1-pve3"
-tmux set-window-option -t netssh-pve synchronize-panes off
+# Keys as typed in xterm (send-keys -K goes through the client's prefix and
+# key tables): prefix e excludes the active pane, prefix S toggles all panes.
+client=$(tmux list-clients -t netssh-pve -F '#{client_name}' | head -1)
+press() { tmux send-keys -K -c "$client" C-b "$1"; sleep 0.5; }
+panes=($(tmux list-panes -t netssh-pve -F '#{pane_id}'))
+tmux select-pane -t "${panes[2]}"
+press e
+visible "tmux: prefix e marks the pane [SOLO]" tmux-solo "$p3 [SOLO]" "$p1 [SYNC]" "$p2 [SYNC]"
+tmux send-keys -t "${panes[2]}" "echo solo-only" Enter
+tmux send-keys -t "${panes[0]}" "echo two-panes" Enter
+sleep 1
+shot tmux-solo-2
+[ "$(screen_count tmux-solo-2 "solo-only")" = 2 ] && ok "tmux: the excluded pane types alone" || bad "tmux: the excluded pane types alone" "$(screen_text tmux-solo-2)"
+[ "$(screen_count tmux-solo-2 "two-panes")" = 4 ] && ok "tmux: broadcast skips the excluded pane" || bad "tmux: broadcast skips the excluded pane" "$(screen_text tmux-solo-2)"
+press e
+visible "tmux: prefix e again includes it" tmux-solo-3 "$p3 [SYNC]"
+
+press S
+sync_is() { [ "$(tmux show-window-options -t netssh-pve -v synchronize-panes 2>/dev/null)" = "$1" ]; }
+wait_for 5 sync_is off && ok "tmux: prefix S turns broadcast off" || bad "tmux: prefix S turns broadcast off" "$(tmux show-window-options -t netssh-pve)"
 tmux send-keys -t netssh-pve "echo just-this-pane" Enter
 sleep 1
 shot tmux-3
 [ "$(screen_count tmux-3 "just-this-pane")" = 2 ] && ok "tmux: without broadcast only one pane gets input" || bad "tmux: without broadcast only one pane gets input" "$(screen_text tmux-3)"
 shows_sync() { screen_shows tmux-3 "[SYNC]"; }
 shows_sync && bad "tmux: [SYNC] disappears when broadcast is off" "$(screen_text tmux-3)" || ok "tmux: [SYNC] disappears when broadcast is off"
+press S
+wait_for 5 sync_is on && ok "tmux: prefix S turns it on again" || bad "tmux: prefix S turns it on again" "$(tmux show-window-options -t netssh-pve)"
 tmux kill-server
 
 # --- more hosts than panes_per_window: further windows ---
@@ -64,6 +85,12 @@ zellij kill-all-sessions --yes >/dev/null 2>&1
 # --- TUI: mark hosts, open them together ---
 term tmux new-session -s tui netssh
 visible "TUI: search screen" tui-1 "Search hostname"
+# The netssh keys do nothing outside netssh multi windows.
+tmux source-file "$XDG_CACHE_HOME/netssh/netssh-multi.tmux.conf"
+client=$(tmux list-clients -t tui -F '#{client_name}' | head -1)
+tmux send-keys -K -c "$client" C-b S
+sleep 0.5
+[ "$(tmux show-window-options -t tui -v synchronize-panes 2>/dev/null)" != on ] && ok "tmux: prefix S leaves other windows alone" || bad "tmux: prefix S leaves other windows alone" "synchronize-panes turned on in a non-netssh window"
 tmux send-keys -t tui "pve"
 visible "TUI: results listed" tui-2 "$p1" "$p2" "$p3"
 tmux send-keys -t tui C-a