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_elevation.luau

@@ -0,0 +1,86 @@
+--[[
+	diag_elevation.luau -- measure the vertical relationship between the wangan
+	mainline and the Daikoku PA, so the "elevate the highway over the PA"
+	rebuild can be designed from real numbers instead of guesses.
+
+	Reports, for the stretch of mainline nearest the PA:
+	  * the deck Y profile (is it flat? already climbing? in a tunnel?)
+	  * which SIDE of the carriageway the PA sits on, per sample
+	  * perpendicular distance from the mainline centreline to the PA centre
+	  * terrain/ground height under the PA and under the mainline
+
+	Read-only.
+]]
+
+local RS = game:GetService("ReplicatedStorage")
+local Workspace = game:GetService("Workspace")
+local Circuit = require(RS:WaitForChild("Shared"):WaitForChild("Circuit"))
+
+local samples = Circuit.samples
+local c = Circuit.PA_CENTER
+local n = #samples
+
+-- Nearest non-tunnel sample = where planPA anchors the junctions.
+local bestIdx, bestD = 1, math.huge
+for i, s in ipairs(samples) do
+	local d = (Vector3.new(s.pos.X, 0, s.pos.Z) - Vector3.new(c.X, 0, c.Z)).Magnitude
+	if not s.tunnel and d < bestD then
+		bestIdx, bestD = i, d
+	end
+end
+
+-- Ground probe: ray straight down, ignoring the built map so we hit terrain.
+local map = Workspace:FindFirstChild("Map")
+local rp = RaycastParams.new()
+rp.FilterType = Enum.RaycastFilterType.Exclude
+rp.FilterDescendantsInstances = map and { map } or {}
+
+local function groundY(x, z, fromY)
+	local hit = Workspace:Raycast(Vector3.new(x, fromY, z), Vector3.new(0, -4000, 0), rp)
+	return hit and math.floor(hit.Position.Y * 10) / 10 or nil
+end
+
+local rows = {}
+for k = -30, 30 do
+	local i = ((bestIdx + k - 1) % n) + 1
+	local s = samples[i]
+	local nxt = samples[(i % n) + 1]
+	local fwd = (nxt.pos - s.pos)
+	fwd = Vector3.new(fwd.X, 0, fwd.Z)
+	if fwd.Magnitude < 0.01 then
+		continue
+	end
+	fwd = fwd.Unit
+	local right = Vector3.new(-fwd.Z, 0, fwd.X)
+	local toPA = Vector3.new(c.X - s.pos.X, 0, c.Z - s.pos.Z)
+	table.insert(rows, {
+		k = k,
+		i = i,
+		y = math.floor(s.pos.Y * 10) / 10,
+		tun = s.tunnel and 1 or 0,
+		-- +1 => PA is to the RIGHT of travel, -1 => LEFT
+		side = (toPA.Unit:Dot(right) >= 0) and 1 or -1,
+		-- signed perpendicular offset and along-track offset to the PA centre
+		perp = math.floor(toPA:Dot(right)),
+		along = math.floor(toPA:Dot(fwd)),
+		dist = math.floor(toPA.Magnitude),
+	})
+end
+
+local ys = {}
+for _, r in ipairs(rows) do
+	table.insert(ys, r.y)
+end
+table.sort(ys)
+
+return game:GetService("HttpService"):JSONEncode({
+	bestIdx = bestIdx,
+	bestDist = math.floor(bestD),
+	paY = c.Y,
+	mainYmin = ys[1],
+	mainYmax = ys[#ys],
+	groundUnderPA = groundY(c.X, c.Z, c.Y + 500),
+	groundUnderMain = groundY(samples[bestIdx].pos.X, samples[bestIdx].pos.Z, samples[bestIdx].pos.Y + 500),
+	ringRadius = (require(RS.Shared.Circuit) and Circuit.PA_RADIUS + 130) or 0,
+	rows = rows,
+})