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_line277.luau

@@ -0,0 +1,67 @@
+--[[
+	diag_line277.luau -- the build harness died at CarManager:277 with
+	"attempt to concatenate string with nil", but line 277 of the LOCAL file is
+	a comment. So the code that ran is not the code on disk.
+
+	Two candidates:
+	  (a) the pushed Source is out of sync with disk
+	  (b) require() handed back a STALE CACHED module from earlier this session
+	      (the known Studio trap: editing Source does NOT invalidate the cache)
+
+	This reads the Source text directly -- never require() -- and prints the
+	real lines around 277, plus the reverted-convention markers, so we can tell
+	(a) from (b) instead of guessing.
+]]
+
+local RS = game:GetService("ReplicatedStorage")
+local SSS = game:GetService("ServerScriptService")
+
+local function find(name)
+	for _, root in ipairs({ SSS, RS, game:GetService("ServerStorage"), workspace }) do
+		local m = root:FindFirstChild(name, true)
+		if m and m:IsA("ModuleScript") then
+			return m
+		end
+	end
+	return nil
+end
+
+local cm = find("CarManager")
+if not cm then
+	return "FATAL: CarManager not found"
+end
+
+local src = cm.Source
+local lines = {}
+for l in (src .. "\n"):gmatch("([^\n]*)\n") do
+	table.insert(lines, l)
+end
+
+local out = {}
+table.insert(out, string.format("%s  %d bytes, %d lines", cm:GetFullName(), #src, #lines))
+table.insert(out, "")
+table.insert(out, "--- Studio Source lines 270..284 ---")
+for i = 270, math.min(284, #lines) do
+	table.insert(out, string.format("%4d | %s", i, lines[i]))
+end
+
+table.insert(out, "")
+table.insert(out, "--- markers ---")
+table.insert(out, "  isFront = zSign < 0 present : " .. tostring(src:find("isFront = zSign < 0", 1, true) ~= nil))
+table.insert(out, "  _buildForDiag present       : " .. tostring(src:find("_buildForDiag", 1, true) ~= nil))
+
+-- Where does _buildForDiag get assigned, per the Studio copy?
+for i, l in ipairs(lines) do
+	if l:find("_buildForDiag", 1, true) then
+		table.insert(out, string.format("  _buildForDiag at line %d: %s", i, l))
+	end
+end
+
+-- What does buildCarModel expect? Print its declaration line.
+for i, l in ipairs(lines) do
+	if l:find("function buildCarModel", 1, true) or l:find("local function buildCarModel", 1, true) then
+		table.insert(out, string.format("  buildCarModel declared line %d: %s", i, l))
+	end
+end
+
+return table.concat(out, "\n")