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>

.gitignore

@@ -23,3 +23,4 @@ Thumbs.db
 # local test environment (testenv/up.sh)
 testenv/.keys/
 testenv/.state/
+testenv/.artifacts/