Duplicate a unit: describe the furniture once, stamp out the rest

Eric's ask. duplicate-unit clones the STRUCTURE — openings with their
sizes, zones, and descriptions, under fresh ids and the first free
"<name> 2"-style name — never the game assignments. And because a
copy immediately wants a real name, units gained rename: opening ids
are stable through it so locations and export URLs never notice, and
a rename-only edit no longer risks wiping the description (the body
field learned the None-means-leave-alone convention). Tests pin the
disjoint ids, the empty copy, the numbering past taken names, and
the wipe-nothing rename.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016jXZFSTZQKzAC8fqpWSz9g

data/furniture.json

@@ -120,6 +120,52 @@
         }
       ],
       "description": "Centerpiece of the game wall in the library"
+    },
+    {
+      "name": "Library Right Shelving",
+      "description": "To the right of the Kallax",
+      "openings": [
+        {
+          "id": "library-right-shelving-a1",
+          "label": "A1",
+          "zone": "",
+          "width_in": 23.25,
+          "height_in": 10.25,
+          "depth_in": 9.5
+        },
+        {
+          "id": "library-right-shelving-b1",
+          "label": "B1",
+          "zone": "",
+          "width_in": 23.25,
+          "height_in": 10.25,
+          "depth_in": 9.5
+        },
+        {
+          "id": "library-right-shelving-c1",
+          "label": "C1",
+          "zone": "",
+          "width_in": 23.25,
+          "height_in": 10.25,
+          "depth_in": 9.5
+        },
+        {
+          "id": "library-right-shelving-d1",
+          "label": "D1",
+          "zone": "",
+          "width_in": 23.25,
+          "height_in": 10.25,
+          "depth_in": 9.5
+        },
+        {
+          "id": "library-right-shelving-e1",
+          "label": "E1",
+          "zone": "",
+          "width_in": 23.25,
+          "height_in": 10.25,
+          "depth_in": 9.5
+        }
+      ]
     }
   ]
 }