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/scale.md

@@ -39,6 +39,72 @@ The HVAC budget in the utilities CSV already assumes ~1,500 m² conditioned.
 Single story throughout: all heavy equipment needs ground-slab floor
 loading (multiple tons per m²).
 
+### Detached utility yard, parking, and what that moves
+
+The massing model puts the boiler and chiller in a **detached utility yard**
+alongside the main building rather than inside it, which is what the boiler's
+own-room requirement in [facility security](facility-security.md) and biomass
+fuel delivery both push toward:
+
+| | Area | Height |
+|---|---|---|
+| Boiler hall | 66 m² | 6.0 m |
+| Biomass fuel store | 42 m² | 5.0 m |
+| Chiller room | 60 m² | 5.0 m |
+| Cooling tower (outdoor pad) | 10 m² | 3.5 m |
+| Flue stack | — | 14 m |
+
+Boiler plus chiller at 126 m² lands inside the 100 to 150 m² the zone table
+allows — but moving them out of the main shed frees that floor for
+production, so either the main building shrinks by ~126 m² or the program
+grows into it. Decide which before the building is priced.
+
+Staff parking is roofed, in the front yard clear of the loading route:
+**10 car bays** (25 × 6 m canopy) and **~30 scooter spaces** (14 × 5 m),
+carrying a further **~32 kWp** of PV on their canopies. At 4.1 m and 3.5 m
+they sit below the main building's 7.0 m low eave, so they cannot shade it.
+
+### Expansion headroom: a second building
+
+Tested against the candidate Mueang Chi parcel, in the massing model at
+`factory_building_rough.FCStd`. The whole zone table above fits inside one
+60 × 25 m building, so a second one is expansion **beyond** the current
+program — doubling pressing capacity, or moving finished goods out to free
+production floor — not a gap in it.
+
+| | Phase 1 | Phase 2 | Total |
+|---|---|---|---|
+| Footprint | 60 × 25 m — 1,500 m² | 45 × 38 m — 1,710 m² | 3,210 m² |
+| Roof PV | ~247 kWp | ~251 kWp | **~498 kWp** |
+| Tightest boundary clearance | 15.7 m | 3.0 m | |
+
+- **45 × 38 m is the sweet spot.** The plot narrows going south, so past
+  ~45 m of length the width falls away faster than length adds: 40 m gives
+  1,607 m², 45 m gives 1,723 m², 50 m drops to 1,558 m², and beyond 65 m
+  nothing useful fits.
+- **A 20 m yard** between the buildings covers fire separation and shared
+  loading off the phase 1 circulation road.
+- **Roof form: two-bay sawtooth, shaded rows deliberately left empty.**
+  A single 38 m monopitch would be the best solar answer — 28 panel rows,
+  none shaded, ~17% more yield than what is drawn — but it needs a **13.7 m**
+  high wall. The sawtooth holds the high side to **10.35 m**, at a cost: each
+  bay falls toward ESE onto the next bay's 3.35 m riser, so the bottom rows
+  sit in shadow (the lowest is blocked below a 53° sun profile angle). Those
+  two rows on the first bay are left empty rather than populated, which
+  trades ~8% of the roof's yield for clean strings, no mismatch losses, and
+  a maintenance walkway. Building height won this trade; revisit it if
+  cladding a 13.7 m wall turns out cheaper than the lost generation.
+  A 7° single pitch (11.7 m high side) sits between the two if wanted.
+- **Coverage stays comfortable**: 3,210 m² is 39% of the deed area (34% of
+  the mapped extent), well under the 50 to 60% typically allowed.
+- **The two roofs together clear 500 kWp**, which removes the need for the
+  ground-mount solar assumed further down this page.
+
+Two caveats. Phase 2's tightest corner sits **3.0 m** from the boundary and
+must be checked against the local ผังเมือง setback rule before it is taken as
+real. And the plot's zoning for factory use is still unverified — that gates
+phase 1 every bit as much as phase 2.
+
 ## Land: factory plot
 
 Industrial plots typically allow 50 to 60% building coverage, and the site