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/README.md

@@ -0,0 +1,46 @@
+# Fixture Invoicing
+
+> **⚠ THIS IS A FIXTURE, NOT A PRODUCT.**
+> A deliberately minimal invoicing app used as a test surface for the
+> [Soleprint](../../README.md) framework. Seed data is obviously placeholder
+> ("Acme Widget Co.", "Test Customer 001"). Do not mistake this for real
+> invoicing software.
+
+## What's here
+
+- `backend/` — FastAPI + SQLAlchemy against Postgres
+- `frontend/` — Vue 3 + Vite, three pages (customers, invoices, invoice detail)
+- `docker-compose.yml` — backend + frontend + postgres as three containers
+
+## Entities
+
+```
+Customer ──< Invoice ──< LineItem
+              └──────< Payment
+```
+
+- `Customer` — name, email
+- `Invoice` — number, customer_id (FK), issued_at, due_at, status
+- `LineItem` — invoice_id (FK), description, quantity, unit_price
+- `Payment` — invoice_id (FK), amount, method, paid_at
+
+## Run it standalone
+
+```bash
+cd examples/fixture-invoicing
+docker compose up --build
+```
+
+- Backend: http://localhost:8100
+- Frontend: http://localhost:3100
+- Postgres: localhost:5532 (user `postgres`, password `fixture`, db `fixture`)
+
+## Run it wrapped by soleprint
+
+The `cfg/sample/` room wraps this fixture. See the main soleprint docs.
+
+```bash
+cd ../..               # back to repo root
+python build.py --cfg sample
+cd gen/sample && ./ctrl/start.sh
+```