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/client.Dockerfile

@@ -14,7 +14,7 @@ FROM alpine:3.23
 # xvfb/xterm/imagemagick/tesseract: render panes in a real terminal emulator,
 # take screenshots, and read them back with OCR (tests/16-render.sh).
 RUN apk add --no-cache openssh-client-default tmux zellij bash zsh fish bash-completion \
-      curl python3 jq ncurses coreutils \
+      curl python3 jq ncurses coreutils grep \
       xvfb xterm imagemagick tesseract-ocr tesseract-ocr-data-eng font-dejavu \
  && adduser -D -s /bin/bash tester
 COPY --from=build /out/netssh /usr/local/bin/netssh