Let multiple demos run concurrently without colliding

[JonJagger]

Each repo's demo brought up the same stack on fixed canonical host
ports (saver 4537, web 3000, ...), a hardcoded cyber-dojo network
name, and fixed container names. So a demo in one repo (e.g. web)
could not run while a demo in another (e.g. differ) was up: they
fought over the same host ports and shared network. The workaround
was to let the new demo nuke everything with docker rm -f.

Make each demo its own docker-compose project instead:

• Drop the hardcoded network name and per-service container_name so Compose namespaces them by project (COMPOSE_PROJECT_NAME, default web). Override it to run a second web demo alongside the first.
• Stop publishing web and saver to the host; only nginx needs a host port (CYBER_DOJO_NGINX_HOST_PORT). Backend services reach each other over the project's private network, so nothing else collides. asset_builder is left alone (not part of the demo; build_assets.sh needs its publish).
• Replace demo.sh's global docker rm -f with a project-scoped compose down, so bringing up one demo no longer kills the others.

Container references in the helper scripts no longer assume fixed
names; they resolve by compose project+service label via a shared
service_container() in lib.sh. This also fixes copy_in_saver_test_data
which previously grepped the web_saver substring of the old name.

Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com