test: check what a real terminal renders for multi, zellij, and the TUI

tests/16-render.sh runs tmux, zellij, and the TUI in xterm on Xvfb,
takes screenshots, and reads them back with tesseract: remote prompts
in all panes, borders and titles with the hosts, [SYNC] appearing and
disappearing, panes side by side, broadcast output in every pane and
input in one pane only without it, windows split by panes_per_window,
the status bar, and TUI marks. Screenshots stay in testenv/.artifacts.

OCR reads three preprocessing variants (plain, stretched for dim gray,
brightest channel for colored text) and normalizes characters that
DejaVu Sans Mono makes ambiguous (1/l, C/().

The screenshots showed tmux cutting the session name in the status bar
to "[netssh-pv"; netssh multi now widens status-left-length for its
own session.

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

testenv/client.Dockerfile

@@ -11,8 +11,11 @@ COPY internal ./internal
 RUN CGO_ENABLED=0 go build -o /out/netssh ./cmd/netssh
 
 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 \
+      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
 COPY testenv/client-entrypoint.sh /entrypoint.sh