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>

examples/fixture-invoicing/docker-compose.yml

@@ -0,0 +1,54 @@
+name: fixture_invoicing
+
+services:
+  db:
+    image: postgres:16-alpine
+    container_name: fixture_db
+    environment:
+      POSTGRES_DB: fixture
+      POSTGRES_USER: postgres
+      POSTGRES_PASSWORD: fixture
+    volumes:
+      - pgdata:/var/lib/postgresql/data
+    ports:
+      - "5532:5432"
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U postgres -d fixture"]
+      interval: 5s
+      timeout: 3s
+      retries: 10
+
+  backend:
+    build:
+      context: ./backend
+    container_name: fixture_backend
+    environment:
+      DB_HOST: db
+      DB_PORT: 5432
+      DB_NAME: fixture
+      DB_USER: postgres
+      DB_PASSWORD: fixture
+      SEED_ON_START: "true"
+    ports:
+      - "8100:8000"
+    depends_on:
+      db:
+        condition: service_healthy
+
+  frontend:
+    build:
+      context: ./frontend
+    container_name: fixture_frontend
+    environment:
+      VITE_API_URL: http://localhost:8100
+    ports:
+      - "3100:5173"
+    depends_on:
+      - backend
+    volumes:
+      - ./frontend/src:/app/src
+      - ./frontend/index.html:/app/index.html
+      - ./frontend/vite.config.js:/app/vite.config.js
+
+volumes:
+  pgdata: