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/SCRIPTING.md

@@ -293,6 +293,48 @@ prefabs work together through the document, not through a parent object:
 - Ground-level floors and rugs go to the `floor` layer; upper floors are slabs
   whose top is at the object's z. Furniture starts 0.02 m above its z.
 
+## Data centres
+
+`datacentre.rhai` lays out multi-storey data centres from parts that read
+each other through the document, like house parts:
+
+- `dc_building` is the shell: `storeys` of `storey_h` (the project
+  parameter; 5 m suits data halls), outer walls split into pieces of at most
+  15 m, a slab per storey and a flat roof with a parapet. On level `kf` the
+  storeys below are closed, the walls facing the viewer are cut 1.2 m above
+  storey k, the others reach its ceiling, and everything above is left out.
+- `part_hidden(o)` (house.rhai) leaves out a part above the storey a level
+  cuts open, and a part on a closed storey inside a `dc_building`: the shell
+  hides it, and drawing it would only make sort cycles with what stands on
+  the slab above. House walls, doors, floors, stairs, furniture, people and
+  every data centre part use it (`dc_skip` is the same rule).
+- `dc_lift(o)`: the top of a `raised_floor` under o on its storey; racks,
+  pods, CRAH units, cabinets, gas banks, devices and people stand on it.
+- A rack line is one sprite with a box per cabinet (`seq`) and the faces
+  painted with `open()` per class (equipment, LED strips, rear PDUs), so a
+  pod of 20 racks costs a few dozen nodes. In-row coolers (`inrow`) are
+  0.3 m units between groups of racks. A pod is three sprites (two rows and
+  the aisle between them: vent tiles, airflow puffs, end doors, roof, duct),
+  which never overlap.
+- Overhead services (`cable_tray`, `busway`, `fire_pipe`, `smoke_detectors`)
+  draw in the layer `ceiling` (`dc_ceiling_layer()`: sorted, z first) after
+  everything on the floor, with sort boxes as tight as the run itself
+  (hangers do not count): a tray beside a pod with a busway over the pod and
+  under the tray has no valid order as boxes in one layer. Keep runs that
+  cross or lie side by side at heights that do not overlap. The project
+  parameter `ceiling` = false leaves them out.
+- Totals: `dc_it(storey)` (IT load, racks; every storey for -1),
+  `dc_cool(storey)` (CRAH duty and standby, in-row coolers),
+  `dc_redundancy(cooling, kw)` ("N+1, 140 kW spare"), `dc_plant()` (chillers
+  and towers), `dc_agent(agent, V, t, litres)` (Novec 1230 at 4.5 %, FM-200
+  at 7 % by NFPA 2001 / ISO 14520, Inergen at 39.9 %), `dc_gas_volume(o)`.
+  A gas bank that is short puts a `warntag` on the view.
+- Wall devices facing away from the viewer on the storey cut open hang on
+  the far side of a wall that is cut low: they draw ghosted.
+
+`isokit check` names where the depth sort broke a cycle ("a before b, c":
+sprite ids, `Rendered.cycle_at`), which points at the sprites to split.
+
 ## Rules that keep the depth sort right
 
 These are lessons from the CNX/Mooban views, and the engine relies on them: