add fixture-invoicing example, sample-room wrap, kind cluster support

- examples/fixture-invoicing/: FastAPI + Vue + Postgres demo (4-entity invoice fixture)
- cfg/sample/: wraps the fixture (managed.repos points at examples/)
- ctrl/kind-{up,down,status}.sh + per-room k8s render in soleprint/ctrl/k8s/
- build.py: relative repo paths, resilient rmtree, optional k8s render hook
- cfg/.gitignore: stop ignoring sample/ and standalone/ template rooms

Manifests render cleanly but kind cluster has not been run end-to-end yet.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cfg/sample/soleprint/docker-compose.nginx.yml

@@ -21,7 +21,7 @@ services:
     image: nginx:alpine
     container_name: ${DEPLOYMENT_NAME}_nginx
     ports:
-      - "80:80"
+      - "${NGINX_PORT:-80}:80"
     volumes:
       - ./nginx/local.conf:/etc/nginx/conf.d/default.conf:ro
     networks: