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>

CHANGELOG.md

@@ -6,8 +6,9 @@
 - Fix `netssh sftp` and `netssh scp` with instance-qualified hosts (`prod/web01:/path`), which were taken for local paths.
 - Fix shell aliases of instance-qualified hosts ignoring anchored rewrite rules (`fsn_fsn1-pve1` instead of `fsn_fsnpve1`).
 - Add `netssh multi` to open several hosts side by side in tmux or zellij: host lists, `tag:`, NetBox virtualization clusters (`cluster:`, devices by default, `--kind vm|all`), and groups from the config (`@name`). `--sync` sends typing to all panes; `netssh sync` toggles it later. In the TUI, `Space` marks hosts and `Enter` opens them together. Configurable in `netssh configure` (multiplexer, panes per window, groups).
+- `netssh multi` widens the status bar of its own tmux session, which tmux otherwise cuts to `[netssh-pv`.
 - Cache entries remember the NetBox cluster of a host, for `cluster:` completion and offline use.
-- Add `testenv/`, a local Docker environment with two NetBox instances, SSH targets, a jump host, tmux, zellij, and bash/zsh/fish, and 160 end-to-end checks of all features, including `netssh multi` with real tmux and zellij (`testenv/up.sh`, `testenv/test.sh`). Not part of CI/CD.
+- Add `testenv/`, a local Docker environment with two NetBox instances, SSH targets, a jump host, tmux, zellij, and bash/zsh/fish, and 178 end-to-end checks of all features, including `netssh multi` with real tmux and zellij and screenshot checks of what a real terminal (xterm) renders (`testenv/up.sh`, `testenv/test.sh`). Not part of CI/CD.
 
 ## v0.0.23