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>

README.md

@@ -1085,6 +1085,7 @@ Both are non-blocking — the refresh runs as a background process and never del
 | Aliases missing in a new shell | `netssh hook install`, then a new terminal. The cache must not be empty. |
 | A host is named `sftp`, `scp`, `tunnel`, `hosts`, `multi`, `sync`, or like another subcommand | Use `netssh -- sftp`. |
 | `netssh multi needs tmux or zellij` | Install one of them (`apt install tmux`, `brew install tmux`). |
+| zellij shows empty boxes in its tab and status bar | Those are Powerline arrows; use a Nerd Font in your terminal, or add `simplified_ui true` to `~/.config/zellij/config.kdl`. |
 | `cluster:x matches no hosts` | Clusters select devices by default; VMs need `--kind vm` or `--kind all`. The name is matched case-insensitively. |
 | `netssh multi` panes use another config | Inside an existing tmux/zellij session, panes get your `HOME` and `XDG_*` paths, but not `NETSSH_*` variables (they would be visible to other users in the process list). Put such settings in the config file. |
 
@@ -1121,7 +1122,7 @@ End-to-end tests run against real services in a local Docker environment — two
 
 ```sh
 testenv/up.sh        # build and start (first start ~6 min)
-testenv/test.sh      # 160 checks: connect, strategies, instances, shortcuts, rules, sftp/scp, tunnels, notes, shells, TUI, configurator, multi with tmux and zellij
+testenv/test.sh      # 178 checks: connect, strategies, instances, shortcuts, rules, sftp/scp, tunnels, notes, shells, TUI, configurator, multi with tmux and zellij, and what xterm renders (screenshots + OCR)
 testenv/down.sh
 ```