Add site massing model for the Mueang Chi candidate parcel

Models the plot and everything on it in FreeCAD, and writes up the
decisions the modelling forced.

Parcel geometry is traced from the DOL LandsMaps cadastral polygon and
reconciled against that portal's own measured edge lengths (40.86 + 26.61
/ 166.81 / 42.48 + 14.94 / 152.46 m), which the trace reproduces to within
about 1% with 1.67 m of closure error over a 444 m perimeter. Note the
mapped extent comes out near 9,570 sqm against a deed area of 8,266 sqm,
a ~16% gap that needs a surveyor before any offer.

On the site: 60x25 m production building on a 10 degree monopitch, a
45x38 m phase-2 building, roof and carport PV, fire and raw water tanks,
a detached boiler/chiller utility yard, perimeter wall and gate, guard
house, roofed staff parking, and the loading and fire-access roads.

Docs:
- scale.md: expansion headroom for a second building, and the detached
  utility yard, which frees ~126 sqm inside the main shed
- sustainability.md: roof orientation study (rotating the building for
  solar gains under 1 point and costs 4 m of clearance, so it stays
  aligned to the plot), and a new water supply section
- facility-security.md: fire water supply behind the existing
  suppression design

Figures marked as estimates are planning-grade, not engineering
calculations; the PV module spec is a generic placeholder, and zoning for
factory use on this plot is still unverified.

docs/facility-security.md

@@ -103,6 +103,36 @@ occupied spaces. The design position is zoned suppression:
 - **Galvanics**: low fire load (aqueous baths); local clean-agent
   protection for rectifier cabinets is sufficient.
 
+### Fire water supply
+
+Sprinklers, water mist, and hose reels are only as good as the water behind
+them. Mains pressure and flow in Mueang Chi cannot be assumed adequate for a
+sprinkler demand, so the site carries its own reserve and pumps.
+
+- **Stored reserve: ~200 m³**, in a dedicated fire water tank (planning
+  figure: 9 m diameter × 4 m, roughly 204 m³ usable above the pump suction).
+  Basis is an Ordinary Hazard Group 2 sprinkler demand of about 1,100 L/min
+  over the design area, plus a 950 L/min hose-stream allowance, held for 90
+  minutes. **This is a planning estimate, not a fire-engineering
+  calculation** — the real number comes out of the hydraulic design.
+- **Dedicated, not shared**: the reserve is not drawn on by process or
+  sanitary water, and the tank refills automatically.
+- **Fire pump set**: duty pump, standby pump, and a jockey pump holding
+  ring-main pressure, in a pump house beside the tank. The duty pump needs a
+  power path that survives a plant trip (diesel drive, or a supply ahead of
+  the main breaker) — the BESS is not a substitute.
+- **Ring main and hydrants** around the building, laid out so an appliance
+  can reach any elevation, with the yard kept clear for access.
+- **Siting**: the tank stands in the front yard between the road and the
+  building, reachable directly from the frontage without entering the
+  production yard. At 4 m tall it sits well below the building's 7 m low
+  eave, so it never shades the roof PV.
+
+**Warehouse racking height drives the real number.** If finished-goods
+storage goes above roughly 3.7 m, the commodity classification and sprinkler
+design change, and the stored volume grows with them. Fix the racking height
+before the fire design is frozen, not after.
+
 The property insurer will drive much of the final specification, so involve
 them before the design is fixed. The archive gas system and the BESS
 detection affect room layout and belong in the site design brief early.