Add reference-anchored concept generation

docs/screens/concepts/hollywood_boulevard/hollywood_boulevard_entrance_day-v2.refgen.json

@@ -0,0 +1,12 @@
+{
+  "concept": "hollywood_boulevard_entrance_day",
+  "generated": "hollywood_boulevard_entrance_day-v2.png",
+  "model": "google/gemini-2.5-flash-image",
+  "reference": {
+    "file": "docs/screens/refs/hollywood/el-capitan-facade-day-wikimedia-2009-10-27.jpg",
+    "licence": "CC BY 2.0",
+    "page": "https://commons.wikimedia.org/wiki/File:El_Capitan_Theater_frontal_fachade-The_Masonic_Temple.jpg",
+    "licence_checked": true
+  },
+  "suffix": "Use the attached reference photograph as the fixed source of geometry and composition: keep the building's proportions, column rhythm, band heights, signage placement and camera angle exactly as photographed. Only restage lighting, weather, people and surface condition where the prompt asks for it. Do not invent extra storeys, wings or ornaments that the photo does not show."
+}