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/README.md

@@ -90,7 +90,11 @@ xterm on a virtual X server (Xvfb) — takes screenshots, and reads them back
 with OCR (tesseract). It checks what is actually drawn: remote prompts in all
 panes, borders and titles with the host names, `[SYNC]` appearing and
 disappearing, panes side by side, broadcast output in every pane, input in one
-pane only without broadcast, the status bar, and the TUI marks.
+pane only without broadcast, the status bar, and the TUI marks. The broadcast
+keys are pressed as a user would: `tmux send-keys -K` delivers them to the
+xterm client, so they pass through the prefix and key tables (`prefix e` →
+`[SOLO]` and the pane drops out of the broadcast; `prefix S` toggles all panes;
+other windows stay unaffected).
 
 ```sh
 testenv/test.sh render          # then look at the screenshots: