fix(demo): make validation suite runnable and shellcheck-clean

validate-all.sh aborted after the very first check because its
post-increment counters (`((VAR++))`) return exit status 1 when the old
value is 0, and `set -e` terminated the script. Use arithmetic
assignment so the suite runs to completion.

Add a .shellcheckrc to suppress the SC1090/SC1091 warnings that are
inherent to the environment-driven design (dynamic `source` of demo.env),
and drop an unused variable in demo-test.sh, so every script passes
shellcheck cleanly.

💘 Generated with Crush

Assisted-by: Crush:glm-5.2

demo/scripts/demo-test.sh

@@ -74,7 +74,7 @@ test_service_health() {
     log_test "Service health"
     local unhealthy=0
     while IFS= read -r line; do
-        local name status
+        local name
         name=$(echo "$line" | awk '{print $1}')
         [[ "$name" == "NAMES" || -z "$name" ]] && continue
         if echo "$line" | grep -q "(healthy)"; then