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
