Daikoku PA: lift mainline over the apron, rebuild both ramps

The PA ring road and the expressway mainline were both at ground level, so
the ring parapet stood INSIDE the carriageway (measured -10.0 studs of
vertical overlap: a wall in the middle of the road).

Reference photos of the real Daikoku Futo PA show the expressway flying
OVER the parking apron on pillars, with the PA at ground level and ramps
climbing up to meet it. Rebuilt to match:

- mainline lifted 38 studs through the junction, eased in/out (max 5.26%
  grade, within real expressway limits)
- PA ring + apron stay at ground level; legacy PARingPillar props removed
- both on/off ramps regraded over the full rise with smoothstep easing,
  segments overlapped 3 studs so wheels can't catch a seam, plus support
  pillars and edge kerbs where the deck is high

Verified in-engine against built parts (tools/diag_daikoku.luau), not just
source: clearance now +28.0 studs, both ramps continuous (max step 5.62,
no gaps). tools/plot_daikoku.py renders the plan/section for comparison.

tools/diag_seatdir.luau

@@ -0,0 +1,120 @@
+--[[
+	diag_seatdir.luau -- decide, empirically, which way the car DRIVES.
+
+	The contradiction: CarManager:158 says `isFront = zSign < 0  -- car forward
+	is -Z`, so the front axle is built at local -Z. But the VehicleSeat's
+	LookVector points +Z (measured: seatLook.dot(bodyLook) = 1.00).
+
+	In Roblox a VehicleSeat applies throttle along its OWN +Z LookVector, so
+	positive throttle drives the car toward +Z -- i.e. toward the axle the code
+	labels REAR. If that holds, every car drives "backwards" and the shell,
+	which is aligned to the chassis convention, renders nose-first into the
+	direction of travel... backwards on screen.
+
+	This does not guess: it reads the seat's actual Throttle/steer wiring and
+	compares the seat basis against the wheel labels, then reports which axle
+	leads under positive throttle.
+
+	It also checks whether the steering servo lives on the axle that leads,
+	because a car steering with its TRAILING axle is the classic shopping-cart
+	castering symptom the user reported.
+]]
+
+local out = {}
+local function log(...)
+	local parts = {}
+	for i = 1, select("#", ...) do
+		parts[#parts + 1] = tostring((select(i, ...)))
+	end
+	table.insert(out, table.concat(parts, " "))
+end
+
+for _, model in ipairs(workspace:GetDescendants()) do
+	if not (model:IsA("Model") and model.Name:match("^Car_")) then
+		continue
+	end
+
+	local body = model:FindFirstChild("Body")
+	local seat = nil
+	for _, d in ipairs(model:GetDescendants()) do
+		if d:IsA("VehicleSeat") then
+			seat = d
+			break
+		end
+	end
+	if not (body and body:IsA("BasePart") and seat) then
+		continue
+	end
+
+	log("=== " .. model.Name .. " ===")
+	local inv = body.CFrame:Inverse()
+
+	-- Seat look expressed in BODY-LOCAL coords: which local axis does the car
+	-- drive along under positive throttle?
+	local seatLookLocal = inv:VectorToObjectSpace(seat.CFrame.LookVector)
+	log(
+		string.format(
+			"  seat look in body-local = (%.2f, %.2f, %.2f)",
+			seatLookLocal.X,
+			seatLookLocal.Y,
+			seatLookLocal.Z
+		)
+	)
+	local drivesTowardPosZ = seatLookLocal.Z > 0
+	log(string.format("  => positive throttle drives toward local %sZ", drivesTowardPosZ and "+" or "-"))
+
+	-- Where are the labelled axles, and which one has the steering servo?
+	local fZ, fN, rZ, rN = 0, 0, 0, 0
+	local steerAxleZ, steerCount = 0, 0
+	for _, d in ipairs(model:GetDescendants()) do
+		if d:IsA("BasePart") and d.Name:match("^Wheel_") then
+			local z = (inv * d.CFrame).Z
+			if d.Name:sub(7, 7):upper() == "F" then
+				fZ, fN = fZ + z, fN + 1
+			else
+				rZ, rN = rZ + z, rN + 1
+			end
+		end
+		-- servo is parented to the HUB, not the wheel -- find it by name
+		if d:IsA("HingeConstraint") and d.ActuatorType == Enum.ActuatorType.Servo then
+			local host = d.Parent
+			if host and host:IsA("BasePart") then
+				steerAxleZ = steerAxleZ + (inv * host.CFrame).Z
+				steerCount = steerCount + 1
+			end
+		end
+	end
+
+	if fN > 0 and rN > 0 then
+		local fm, rm = fZ / fN, rZ / rN
+		log(string.format("  labelled FRONT axle localZ=%.2f | REAR axle localZ=%.2f", fm, rm))
+		local leadingIsFront = (drivesTowardPosZ and fm > rm) or (not drivesTowardPosZ and fm < rm)
+		log(
+			string.format(
+				"  => the axle LEADING under throttle is the one labelled %s",
+				leadingIsFront and "FRONT (correct)" or "REAR  (car drives backwards!)"
+			)
+		)
+	end
+
+	if steerCount > 0 then
+		local sm = steerAxleZ / steerCount
+		log(string.format("  steering servos=%d, mean host localZ=%.2f", steerCount, sm))
+		local steerLeads = (drivesTowardPosZ and sm > 0) or (not drivesTowardPosZ and sm < 0)
+		log(
+			string.format(
+				"  => steered axle %s under throttle %s",
+				steerLeads and "LEADS" or "TRAILS",
+				steerLeads and "(normal car)" or "(SHOPPING-CART castering)"
+			)
+		)
+	else
+		log("  no steering servos found by name scan")
+	end
+	log("")
+end
+
+if #out == 0 then
+	return "No live cars with a Body + VehicleSeat."
+end
+return table.concat(out, "\n")