Scripted demo-stand vacuum: the checklist needs a robot no hardware can provide

The public stand cannot run any Tier-A map integration (no radios, no
robots), which left the whole vacuum section of docs/TESTING.md untestable
outside the owner's home. demo_robot fakes exactly the surface the card
consumes: a Tasshack-shaped map sensor (dict vacuum_position, rooms named
after the plan rooms so auto-calibration has something to match, flipped Y
so the mirror default is meaningful) and a vacuum that drives a serpentine
route through every room in ~3.5 minutes and docks itself.

demo/stand/README.md

@@ -0,0 +1,13 @@
+# Demo-stand extras
+
+Things that live on the public stand (demo.houseplan.tech) but are not part
+of the shipped integration.
+
+- `demo_robot/` — the scripted robot vacuum (docs/VACUUM.md, "demo stand gets
+  a scripted synthetic robot"). Deployed by copying the folder to
+  `custom_components/demo_robot` in both stand seeds and adding `demo_robot:`
+  to configuration.yaml; the dev stand picks it up automatically from this
+  path (`/opt/hp/bin/hp-update-dev.sh`). The rest of the stand-only config
+  (template LQI sensors, alarm helpers, the smoke automation) lives in the
+  seeds on the stand host — see `docs/TESTING-DEMO.md` and the memory note
+  `houseplan-demo-stand`.