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/up.sh

@@ -5,8 +5,8 @@ set -euo pipefail
 cd "$(dirname "$0")"
 compose=(docker compose -f compose.yaml)
 
-mkdir -p .keys .state
-chmod a+rwx .state # written by the seed container's user
+mkdir -p .keys .state .artifacts
+chmod a+rwx .state .artifacts # written by the container user (screenshots in .artifacts)
 if [ ! -f .keys/id_ed25519 ]; then
   ssh-keygen -q -t ed25519 -N '' -C netssh-test -f .keys/id_ed25519
   cp .keys/id_ed25519.pub .keys/authorized_keys