Add fast iteration tooling

nix run .#fast, .#test and .#webui-dev; see DEVELOPING.md.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AeFqV9dZrK6nipHkPfyM4F

scripts/test.sh

@@ -0,0 +1,45 @@
+# Runs VM tests one at a time (they need a lot of memory), says what failed.
+#   router-test NAME...        run these tests
+#   router-test --all          run all of them
+#   router-test -i NAME        build the test's interactive driver and run it:
+#                              a Python prompt where start_all(), machine.succeed(...)
+#                              and test_script() work; boot once, retry steps
+#   router-test --list
+# Logs: ${TMPDIR:-/tmp}/router-test-logs/NAME.log
+vm_tests="basic pppoe vlans onestick rollback dns installer apply setup update image wireguard"
+flake=${FLAKE:-.}
+logs=${TMPDIR:-/tmp}/router-test-logs
+mkdir -p "$logs"
+
+case ${1:-} in
+  --list) echo "$vm_tests" | tr ' ' '\n'; exit 0 ;;
+  -i|--interactive)
+    [ $# -eq 2 ] || { echo "usage: router-test -i NAME" >&2; exit 2; }
+    driver=$(nix build --no-link --print-out-paths "$flake#checks.x86_64-linux.$2.driverInteractive")
+    exec "$driver/bin/nixos-test-driver" --interactive ;;
+  --all) set -- $vm_tests ;;
+  ""|-*) echo "usage: router-test NAME... | --all | -i NAME | --list" >&2; exit 2 ;;
+esac
+
+failed=0
+total=$SECONDS
+for name in "$@"; do
+  log=$logs/$name.log
+  start=$SECONDS
+  printf '%-12s ' "$name"
+  if nix build "$flake#checks.x86_64-linux.$name" --max-jobs 1 -L --no-link > "$log" 2>&1; then
+    printf 'ok      %4ds\n' $((SECONDS - start))
+  else
+    failed=1
+    printf 'FAILED  %4ds   %s\n' $((SECONDS - start)) "$log"
+    # The failing step and what it said, or the build error before any VM ran.
+    if grep -aq '!!! Test "' "$log"; then
+      grep -a -m1 '!!! Test "' "$log" | sed -E 's/^[^>]*> //; s/^/  /' | cut -c1-300
+      grep -a '!!! ' "$log" | sed -n '2,12p' | sed -E 's/^[^>]*> //; s/^/    /' | cut -c1-240
+    else
+      grep -a -E '^error|error:' -A8 "$log" | head -24 | sed 's/^/  /' | cut -c1-240
+    fi
+  fi
+done
+printf '\n%d test(s), %ds\n' $# $((SECONDS - total))
+exit $failed