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>

internal/multiplex/multiplex.go

@@ -22,7 +22,10 @@ type Options struct {
 	Label     string // window, session, and tab name
 	Sync      bool   // type into all panes of a window at once
 	PerWindow int    // panes per tmux window or zellij tab
-	LayoutDir string // where zellij layout files are written
+	LayoutDir string // where zellij layout files and tmux key bindings are written
+	// SyncKey and SoloKey are tmux prefix keys that toggle broadcast for the
+	// window and exclude the current pane from it; empty means no binding.
+	SyncKey, SoloKey string
 	// Hint receives notes for the user before the process is replaced.
 	Hint func(string)
 }
@@ -98,8 +101,30 @@ func Open(kind string, panes []Pane, opts Options, r Runner) error {
 	return fmt.Errorf("unknown multiplexer %q", kind)
 }
 
-// ToggleSync switches broadcast input in the current tmux window or zellij tab.
-func ToggleSync(r Runner) (string, error) {
+// ToggleSync switches broadcast input in the current tmux window or zellij
+// tab; with pane it excludes the current tmux pane from it or includes it again.
+func ToggleSync(r Runner, pane bool) (string, error) {
+	if pane {
+		if r.Getenv("TMUX") == "" {
+			if r.Getenv("ZELLIJ") != "" {
+				return "", errors.New("zellij broadcasts to a whole tab; single panes cannot be excluded")
+			}
+			return "", errors.New("netssh sync --pane works inside tmux")
+		}
+		solo, err := r.Output("tmux", "display-message", "-p", "#{@netssh_solo}")
+		if err != nil {
+			return "", err
+		}
+		for _, argv := range soloToggle(solo == "1") {
+			if _, err := r.Output(argv...); err != nil {
+				return "", err
+			}
+		}
+		if solo == "1" {
+			return "pane included in broadcast", nil
+		}
+		return "pane excluded from broadcast", nil
+	}
 	switch {
 	case r.Getenv("TMUX") != "":
 		current, err := r.Output("tmux", "show-window-options", "-v", "synchronize-panes")