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/station/tools/graphgen/schema.json

@@ -0,0 +1,44 @@
+{
+  "models": {
+    "Customer": {
+      "doc": "A fixture customer (obviously-placeholder names).",
+      "fields": {
+        "id":         {"type": "int", "pk": true},
+        "name":       {"type": "str"},
+        "email":      {"type": "str"},
+        "created_at": {"type": "datetime"}
+      }
+    },
+    "Invoice": {
+      "doc": "An invoice issued to a customer. Owns line items and payments.",
+      "fields": {
+        "id":          {"type": "int", "pk": true},
+        "number":      {"type": "str"},
+        "customer_id": {"type": "FK:Customer"},
+        "issued_at":   {"type": "datetime"},
+        "due_at":      {"type": "datetime", "nullable": true},
+        "status":      {"type": "str"}
+      }
+    },
+    "LineItem": {
+      "doc": "A single billable line on an invoice.",
+      "fields": {
+        "id":          {"type": "int", "pk": true},
+        "invoice_id":  {"type": "FK:Invoice"},
+        "description": {"type": "str"},
+        "quantity":    {"type": "int"},
+        "unit_price":  {"type": "Decimal"}
+      }
+    },
+    "Payment": {
+      "doc": "A payment recorded against an invoice.",
+      "fields": {
+        "id":         {"type": "int", "pk": true},
+        "invoice_id": {"type": "FK:Invoice"},
+        "amount":     {"type": "Decimal"},
+        "method":     {"type": "str"},
+        "paid_at":    {"type": "datetime"}
+      }
+    }
+  }
+}