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_nosecue.luau

@@ -0,0 +1,116 @@
+--[[
+	diag_nosecue.luau -- confirm WHICH cue is lying.
+
+	diag_shellgeom (bodywork asymmetry, wheel-independent) says the built
+	fleet disagrees on facing: rx7_fc +0.550, silv_s15 +0.206, hatch_junk
+	+0.614 all land one way, while hyper_x -0.290 and gtr_r34 -0.362 land the
+	other. Exactly two cars wrong -- matching the user's report.
+
+	The aligner derives facing from each shell's Wheels folder. If a shell's
+	F*/R* wheel labels are swapped relative to its bodywork, the aligner
+	confidently rotates it backwards.
+
+	This cross-checks the wheel cue against two INDEPENDENT cues that cannot
+	both be wrong:
+	  * named bodywork: Hood/Windshield sit at the nose, Boot/Spoiler at the tail
+	  * bodywork asymmetry (as before)
+	If wheels disagree with the bodywork, the wheel labels are the bug.
+]]
+
+local ReplicatedStorage = game:GetService("ReplicatedStorage")
+
+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
+
+log(string.format("%-14s %10s %10s %10s %10s   %s", "car", "wheelZ", "hoodZ", "bootZ", "asym", "wheels-vs-body"))
+log(string.rep("-", 78))
+
+for _, t in ipairs(ReplicatedStorage.CarModels:GetChildren()) do
+	local pivotInv = t:GetPivot():Inverse()
+
+	-- cue 1: wheels
+	local wheelDz = nil
+	local wf = t:FindFirstChild("Wheels")
+	if wf then
+		local fS, fN, rS, rN = 0, 0, 0, 0
+		for _, w in ipairs(wf:GetChildren()) do
+			local p = w:IsA("BasePart") and w or w:FindFirstChildWhichIsA("BasePart", true)
+			if p then
+				local z = (pivotInv * p.Position).Z
+				local h = w.Name:sub(1, 1):upper()
+				if h == "F" then
+					fS, fN = fS + z, fN + 1
+				elseif h == "R" then
+					rS, rN = rS + z, rN + 1
+				end
+			end
+		end
+		if fN > 0 and rN > 0 then
+			wheelDz = fS / fN - rS / rN
+		end
+	end
+
+	-- cue 2: named bodywork
+	local function avgZ(patterns)
+		local s, n = 0, 0
+		for _, p in ipairs(t:GetDescendants()) do
+			if p:IsA("BasePart") then
+				local nm = p.Name:lower()
+				for _, pat in ipairs(patterns) do
+					if nm == pat then
+						s += (pivotInv * p.Position).Z
+						n += 1
+						break
+					end
+				end
+			end
+		end
+		return n > 0 and (s / n) or nil
+	end
+	local hoodZ = avgZ({ "hood", "windshield", "bonnet", "grille" })
+	local bootZ = avgZ({ "boot", "spoiler", "trunk" })
+
+	-- cue 3: asymmetry
+	local sum, wsum, lo, hi = 0, 0, math.huge, -math.huge
+	for _, p in ipairs(t:GetDescendants()) do
+		if p:IsA("BasePart") then
+			local nm = p.Name:lower()
+			if not (nm:find("wheel") or nm:find("tyre") or nm:find("tire") or nm:find("rim")) then
+				local z = (pivotInv * p.Position).Z
+				local v = p.Size.X * p.Size.Y * p.Size.Z
+				sum, wsum = sum + z * v, wsum + v
+				lo, hi = math.min(lo, z), math.max(hi, z)
+			end
+		end
+	end
+	local asym = wsum > 0 and (sum / wsum - (lo + hi) / 2) or nil
+
+	-- Wheels say nose is at sign(wheelDz). Bodywork says nose is where the
+	-- hood is, i.e. sign(hoodZ). They must agree.
+	local verdict = "?"
+	if wheelDz and hoodZ then
+		local wheelNose = wheelDz < 0 and -1 or 1
+		local hoodNose = hoodZ < 0 and -1 or 1
+		verdict = (wheelNose == hoodNose) and "agree" or "*** DISAGREE -- wheel labels swapped ***"
+	end
+
+	log(
+		string.format(
+			"%-14s %10s %10s %10s %10s   %s",
+			t.Name,
+			wheelDz and string.format("%+.2f", wheelDz) or "n/a",
+			hoodZ and string.format("%+.2f", hoodZ) or "n/a",
+			bootZ and string.format("%+.2f", bootZ) or "n/a",
+			asym and string.format("%+.2f", asym) or "n/a",
+			verdict
+		)
+	)
+end
+
+return table.concat(out, "\n")