Payap model: main gate, guard house, boom barrier; truck paving cut down

User, October 2026: add the guard house and the gate; the truck apron is
far too big.

Model (changed live in FreeCAD 26.3):
- main gate as in the phase 1 RFQ: piers and two 5 m sliding leaves (shown
  open, parked inside the wall), 10 m clear (gate_width 8 -> 10 m);
- guard house as in the RFQ: 4 x 3 m (guard room 8.4 m2 + WC 3.6 m2),
  window wall to the gate, guard desk, on the gate's north-west side;
- boom barrier just inside the gate (cars and trucks both pass it);
- truck paving: the 42 x 101 m apron (~4,240 m2) replaced by an 8 m gate
  road (550 m2) and a 32 m deep apron and dock face (1,600 m2) that joins
  the east service road; new params apron_depth, road_w.

Docs: site-candidates Access bullet (~2,150 m2 heavy-duty paving against
1,608 m2 in the Mueang Chi RFQ).

docs/site-candidates.md

@@ -217,8 +217,18 @@ and every equipment envelope is an estimate.
   pumps. The model's 6 m warehouse racking and its larger buildings will
   raise the volume; the hydraulic design sets it.
 - **Access**:
-  - one gate on the south side opens onto a truck apron in front of the
-    dock doors;
+  - one main gate on the south side, as in the
+    [phase 1 RFQ](finance/phase1-construction-rfq.md): piers and two 5 m
+    sliding leaves, 10 m clear;
+  - the guard house beside it on the gate's north-west side, 4 × 3 m with
+    a WC and air conditioning, and a boom barrier just inside the gate
+    that car and truck traffic both pass;
+  - an 8 m gate road to a truck apron and dock face in front of the dock
+    doors. The apron is 32 m deep, about twice the 16.5 m of an
+    articulated truck reversing to a dock (estimate), and joins the
+    service road round the east end. That makes ~2,150 m² of heavy-duty
+    paving, against 1,608 m² for apron, dock face and internal road in
+    the Mueang Chi RFQ;
   - an access road runs along the south boundary to the car park, with
     a footpath to the office;
   - a lane behind the factory serves the yard, the silos, the water