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

docs/guide.md

@@ -118,7 +118,10 @@ top row renders as exactly that — a diagram of the wall, not a list. Create
 units from presets (IKEA Kallax cube 13.25″ × 13.25″ × 15.4″, Billy shelf,
 custom, or a no-size virtual spot like "travel case") as grids of rows ×
 columns; add more sections to an existing unit the same way — a later grid
-continues the row letters, so 3 × 4 under an A row lands as B1…D4. Openings
+continues the row letters, so 3 × 4 under an A row lands as B1…D4. Duplicate a
+unit to stamp out identical furniture — the copy takes the structure
+(openings, sizes, zones, descriptions) under a fresh auto-numbered name,
+never the games — and rename any unit in place. Openings
 are editable (dimensions are all-or-none: three numbers or a no-limit
 spot), reorderable within their row (moves clamp at row boundaries so the
 diagram can't fragment), and deletable. Both units and openings take a