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/frontend/src/App.vue

@@ -0,0 +1,17 @@
+<template>
+  <div class="fixture-banner">
+    ⚠ FIXTURE APP — SOLEPRINT DEMO — NOT A REAL PRODUCT ⚠
+  </div>
+
+  <header class="app-nav">
+    <h1>Fixture Invoicing</h1>
+    <nav>
+      <router-link to="/customers">Customers</router-link>
+      <router-link to="/invoices">Invoices</router-link>
+    </nav>
+  </header>
+
+  <main class="app-main">
+    <router-view />
+  </main>
+</template>