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_shellparts.luau

@@ -0,0 +1,108 @@
+--[[
+	diag_shellparts.luau -- dump the actual part inventory of hatch_junk and
+	hyper_x. diag_nosecue2 found no headlight/taillight/seat on either, so the
+	naming convention must differ. Read-only: list what IS there, plus a purely
+	geometric asymmetry cue that needs no naming at all.
+
+	Geometric cue (naming-independent): on a car, the passenger cabin /
+	windscreen sits BEHIND the nose, so the mass distribution along the
+	longitudinal axis is asymmetric. Cars are also taller at the cabin than at
+	the nose. Reporting mean height per Z-half lets the shape itself indicate
+	which end is the bonnet -- the tall half is the cabin/rear-biased end.
+]]
+
+local RS = game:GetService("ReplicatedStorage")
+local SSS = game:GetService("ServerScriptService")
+local SS = game:GetService("ServerStorage")
+
+local TARGETS = { hatch_junk = true, hyper_x = true, rx7_fc = true }
+
+local out = {}
+local function log(s)
+	table.insert(out, s)
+end
+
+local shells = {}
+for _, root in ipairs({ RS, SSS, SS, workspace }) do
+	for _, d in ipairs(root:GetDescendants()) do
+		if d:IsA("Model") and TARGETS[d.Name] and d:FindFirstChild("Wheels") and not shells[d.Name] then
+			shells[d.Name] = d
+		end
+	end
+end
+
+for _, name in ipairs({ "rx7_fc", "hatch_junk", "hyper_x" }) do
+	local shell = shells[name]
+	if not shell then
+		log(name .. ": NOT FOUND")
+		continue
+	end
+	local pivotInv = shell:GetPivot():Inverse()
+
+	log("=== " .. name .. " ===")
+
+	-- Part names outside the Wheels folder, with local Z, sorted front-to-back.
+	local rows = {}
+	for _, d in ipairs(shell:GetDescendants()) do
+		if d:IsA("BasePart") and not d:IsDescendantOf(shell.Wheels) then
+			local p = pivotInv * d.Position
+			table.insert(rows, { name = d.Name, z = p.Z, y = p.Y, cls = d.ClassName })
+		end
+	end
+	table.sort(rows, function(a, b)
+		return a.z < b.z
+	end)
+
+	log(string.format("  %d non-wheel parts (sorted by local Z, -Z first):", #rows))
+	local shown = 0
+	for _, r in ipairs(rows) do
+		if shown < 24 then
+			log(string.format("    z=%+7.2f  y=%+6.2f  %-28s %s", r.z, r.y, r.name, r.cls))
+			shown += 1
+		end
+	end
+	if #rows > 24 then
+		log(string.format("    ... %d more", #rows - 24))
+	end
+
+	-- Naming-independent shape cue: mean height of parts in each Z half.
+	local negY, negN, posY, posN = 0, 0, 0, 0
+	local minZ, maxZ
+	for _, r in ipairs(rows) do
+		minZ = minZ and math.min(minZ, r.z) or r.z
+		maxZ = maxZ and math.max(maxZ, r.z) or r.z
+	end
+	local midZ = (minZ and maxZ) and ((minZ + maxZ) / 2) or 0
+	for _, r in ipairs(rows) do
+		if r.z < midZ then
+			negY, negN = negY + r.y, negN + 1
+		else
+			posY, posN = posY + r.y, posN + 1
+		end
+	end
+	if negN > 0 and posN > 0 then
+		local negMean, posMean = negY / negN, posY / posN
+		log(
+			string.format(
+				"  shape cue: mean height  -Z half = %+.2f (n=%d)   +Z half = %+.2f (n=%d)",
+				negMean,
+				negN,
+				posMean,
+				posN
+			)
+		)
+		local tallEnd = (negMean > posMean) and "-Z" or "+Z"
+		log(
+			string.format(
+				"             taller (cabin) end = %s  => nose is likely %s",
+				tallEnd,
+				(tallEnd == "-Z") and "+Z" or "-Z"
+			)
+		)
+	else
+		log("  shape cue: unavailable")
+	end
+	log("")
+end
+
+return table.concat(out, "\n")