Data centres: multi-storey shell, racks and pods, cooling, fire safety, power

datacentre.rhai (palette groups Data centre, Cooling, Fire safety, Power):

- dc_building: multi-storey shell (storeys of storey_h), walls in pieces,
  slabs, roof with parapet, louvres, front door and loading dock; levels
  cut it open storey by storey
- raised_floor; rack_row and dc_pod (cabinets by U, width, depth,
  equipment, fill, doors, LEDs; in-row coolers; cold / hot aisle
  containment with vent tiles, roof, end doors, ceiling duct, airflow);
  dc_cage for colocation
- crah, chiller (air-cooled, dry, adiabatic), cooling_tower, pipe_run
- smoke_detectors (grid + coverage), fire_pipe (VESDA, sprinkler, gas),
  gas_bank (Novec 1230, FM-200, Inergen sized for its halls), fire_device
  (panels, call points, abort switches, beacons, extinguishers)
- power_cab (UPS, battery, PDU, switchgear, RPP), busway, cable_tray,
  transformer
- tooltips sum IT load, kW/m2, cooling redundancy per storey, UPS load,
  battery runtime, plant cooling and agent quantity (warning when short)

Engine and stdlib:

- part_hidden(o) in house.rhai: parts on closed storeys inside a
  dc_building are not drawn (they only made sort cycles under the slab
  above); house parts, furniture and people use it. Existing examples
  render byte-identical.
- overhead services sort in their own "ceiling" layer with tight boxes
- sort records which sprites each broken cycle involved; isokit check
  names them (Rendered.cycle_at)

examples/datacentre (three storeys and a roof plant), Data centre demo in
the web Studio, tests, SCRIPTING/README/ROADMAP, MCP instructions.

docs/ROADMAP.md

@@ -71,6 +71,31 @@ Each phase ends with tests, a Studio/viewer check in Edge, commit and push.
 - [x] Diagonal walls (any angle, mitred corners, windows and doors on them)
 - [ ] Openings across two walls; floor openings for stairs
 
+## Data centres (done)
+
+- [x] `dc_building`: multi-storey shell (storeys of `storey_h`), walls in pieces, slabs, roof with
+      parapet, louvres, front door and loading dock; levels cut it open storey by storey, parts on
+      closed storeys inside it are left out (`part_hidden`, also for house parts and people)
+- [x] Raised floor (tiles, void with pedestals); racks, pods, units, cabinets and people stand on it
+- [x] Rack row and Rack pod: cabinets by U, width, depth, equipment (servers, storage, GPU, network,
+      empty), fill, doors, status LEDs; in-row coolers; cold / hot aisle containment (vent tiles,
+      roof, end doors, ceiling duct, airflow); colocation Cage
+- [x] Cooling: CRAH / CRAC units, chillers, dry and adiabatic coolers, cooling towers, pipe runs
+- [x] Fire safety: detector grids with coverage, VESDA / sprinkler / gas discharge pipes, clean-agent
+      cylinder banks sized for their halls (Novec 1230, FM-200, Inergen), panels, call points, abort
+      switches, beacons, extinguishers
+- [x] Power: UPS, battery, PDU, switchgear and RPP line-ups, busways, cable trays, transformers
+- [x] Tooltips do the sums: IT load and kW/m², cooling redundancy per storey, UPS load, battery
+      runtime, plant cooling, agent quantity (warning label when short)
+- [x] Overhead services in their own `ceiling` layer (z first) after the floor; `isokit check` names
+      where the depth sort broke a cycle
+- [x] `examples/datacentre` (three storeys and a roof), Data centre demo in the web Studio
+- [ ] Next: floor openings and a lift / stair core through the slabs; cold aisle airflow from the
+      CRAHs drawn under the raised floor (section cut, Phase 4); generator yard and fuel system
+      parts (day tanks, ATS); security (CCTV, access control, mantraps); leak detection; liquid
+      cooling (CDUs, rear-door heat exchangers, manifolds); a row-by-row power and cooling budget
+      table on the building
+
 ## Backlog (2026-10)
 
 1. ~~Web performance~~ (done, see below)