test: add a local Docker test environment for all features

testenv/ starts two real NetBox 4.7 instances (v2 and v1 token), SSH
targets on public, mgmt, and prod networks, a jump host, an IPv6-only
host, a static host, and a client with ssh, sftp, scp, tmux, zellij,
bash, zsh, and fish. A seed script fills NetBox through the REST API.

141 end-to-end checks cover cache and refresh filters, connecting,
all resolver strategies, multiple instances, shortcuts and rewrites,
SSH options and rules, static hosts, sftp/scp incl. IPv6 and jump
hosts, tunnels, local notes and NetBox comments, completion and shell
hooks, the TUI (driven through tmux), the configurator, and uninstall.

Local only: testenv/up.sh, test.sh, shell.sh, down.sh. Not part of
CI/CD.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

testenv/up.sh

@@ -0,0 +1,26 @@
+#!/usr/bin/env bash
+# Start the netssh test environment: generate the test key, build the images
+# (netssh from the current source), start everything, and seed NetBox.
+set -euo pipefail
+cd "$(dirname "$0")"
+compose=(docker compose -f compose.yaml)
+
+mkdir -p .keys .state
+chmod a+rwx .state # written by the seed container's user
+if [ ! -f .keys/id_ed25519 ]; then
+  ssh-keygen -q -t ed25519 -N '' -C netssh-test -f .keys/id_ed25519
+  cp .keys/id_ed25519.pub .keys/authorized_keys
+fi
+# Throwaway test key: readable by the container user; tests copy it with 0600.
+chmod 644 .keys/id_ed25519 .keys/authorized_keys
+
+"${compose[@]}" build
+"${compose[@]}" up -d --wait --wait-timeout 600 \
+  postgres redis netbox-fsn netbox-lab bastion fsn1-pve1 fsn1-pve2 fsn1-pve3 \
+  web1 web2 lab-db1 lab-web1 legacy-box v6-host client
+"${compose[@]}" run --rm seed
+echo
+echo "Test environment is up."
+echo "  Tests:        testenv/test.sh [FILTER]"
+echo "  Shell:        testenv/shell.sh"
+echo "  NetBox UI:    http://127.0.0.1:18000 (fsn), http://127.0.0.1:18001 (lab) — admin / admin"