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/verify_revert.luau

@@ -0,0 +1,63 @@
+--[[
+	verify_revert.luau -- confirm the reverted convention is in Edit source.
+	Reads .Source text (never require(), which caches for the Studio session).
+	Asserts the NEW-BAD tokens are ABSENT and the ORIGINAL-GOOD tokens PRESENT,
+	so a stale copy fails loudly instead of passing.
+]]
+
+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 out = {}
+local pass, fail = 0, 0
+local function check(label, cond, detail)
+	if cond then
+		pass += 1
+		table.insert(out, string.format("  %-34s OK    %s", label, detail or ""))
+	else
+		fail += 1
+		table.insert(out, string.format("  %-34s FAIL  %s", label, detail or ""))
+	end
+end
+
+local cm = find("CarManager")
+local ch = find("Chassis")
+if not cm or not ch then
+	return "FATAL: CarManager or Chassis not found in Edit"
+end
+
+local cmSrc = cm.Source
+local chSrc = ch.Source
+
+table.insert(out, string.format("CarManager  %d bytes  %s", #cmSrc, cm:GetFullName()))
+check("isFront reverted to zSign < 0", cmSrc:find("isFront = zSign < 0", 1, true) ~= nil, "")
+check("bad zSign > 0 GONE", cmSrc:find("isFront = zSign > 0", 1, true) == nil, "")
+check("shell solver back on -Z", cmSrc:find("atan2(nose.X, -nose.Z)", 1, true) ~= nil, "")
+check("bad solver GONE", cmSrc:find("atan2(-nose.X, nose.Z)", 1, true) == nil, "")
+check("regression warning recorded", cmSrc:find("made things strictly worse", 1, true) ~= nil, "")
+
+table.insert(out, string.format("Chassis     %d bytes  %s", #chSrc, ch:GetFullName()))
+check("DRIVE_SIGN reverted to -1", chSrc:find("DRIVE_SIGN = -1", 1, true) ~= nil, "")
+check("bad DRIVE_SIGN = 1 GONE", chSrc:find("DRIVE_SIGN = 1 ", 1, true) == nil, "")
+
+table.insert(out, "")
+table.insert(out, string.format(">>> %d passed, %d failed", pass, fail))
+if fail > 0 then
+	table.insert(out, ">>> REVERT INCOMPLETE -- do not press Play")
+else
+	table.insert(out, ">>> Revert confirmed in Edit source. Save (Ctrl+S) before Play.")
+	table.insert(out, "    NOTE: this proves the SOURCE is reverted. It does NOT prove the")
+	table.insert(out, "    cars render correctly -- only your eyes in Play can show that.")
+end
+
+return table.concat(out, "\n")