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>

ctrl/kind-up.sh

@@ -0,0 +1,21 @@
+#!/bin/bash
+# Create (or no-op) the single shared `spr` kind cluster used by every
+# soleprint room. Per-room work happens inside namespaces — see
+# `gen/<room>/ctrl/k8s-up.sh`.
+set -e
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+KIND_CONFIG="$SCRIPT_DIR/k8s/kind-config.yaml"
+
+if kind get clusters 2>/dev/null | grep -q '^spr$'; then
+    echo "Kind cluster 'spr' already exists."
+else
+    echo "Creating kind cluster 'spr'..."
+    kind create cluster --config "$KIND_CONFIG"
+fi
+
+kubectl config use-context kind-spr >/dev/null
+
+echo
+echo "Cluster ready. Per-room deploy:"
+echo "  cd gen/<room> && ./ctrl/k8s-up.sh"