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/compose.yaml

@@ -165,6 +165,7 @@ services:
       - ./.keys:/keys:ro
       - ./.state:/state
       - ./tests:/tests:ro
+      - ./.artifacts:/artifacts
     networks:
       public: {ipv4_address: 10.231.0.10, ipv6_address: "fd00:30::10"}
       mgmt: {ipv4_address: 10.231.1.10}