Disable PA on/off ramps; restore mainline to pre-ramp state

Three iterations of the Daikoku PA ramps never read as a real
interchange. Measured why, rather than tuning further:

  * the entry and exit decks INTERSECTED -- 2547 sample pairs within
    30 studs horizontally at <12 studs dY, worst case 0.5 studs apart
    at dY=0.0. On screen that is the bowtie tangle of white rails.
  * both real Daikoku ramps turn the SAME way (clockwise; the guide
    video says "right, right"). This build mirrored them into each
    other.
  * the real arc wraps OUTSIDE the lot and forms its north/east
    boundary. This build cut across the apron interior.

Gated behind BUILD_PA_RAMPS=false rather than deleted -- the radius
was already correct (470 studs vs an OSM-measured mean of 469) and
the merge lands tangent at 0.1-0.2 deg divergence. Both worth keeping.

Also removed two leftovers that would have outlived the ramps:

  * rampOpen punched holes in the mainline's outer parapet, railing
    and kill wall for ramps that no longer exist -- a barrier gap with
    a drop behind it. Sealed via RAMP_EDGE_OPENINGS=false. Verified:
    Barrier/EdgeRail/KillWall = 1646 each = 823 samples x 2 edges,
    0 road samples without a guard within 90 studs.
  * gore zebra hatching painted 1024 bars + 64 boundary lines marking
    a junction that is not there. Sealed via RAMP_GORE_MARKINGS=false.

Verified after a cache-busting fresh rebuild (Studio's require() cache
otherwise serves the previous map and reports a false pass):
  parts 22628 -> 15988
  all ramp part classes = 0
  Road = 823 (unchanged), spawn pad + PASpawn intact
  clip probe TRUSTWORTHY (control +11/-0), CLIPPING=0

Research and rebuild spec kept for when this is revisited:
reference/ramps/BUILD_SPEC.md, INDEX.md (10 CC-licensed Commons
photos), docs/daikoku-reference-spec.md (OSM circle fits).

tools/probe_building_clip.luau

@@ -0,0 +1,108 @@
+-- Building-clip probe, CORRECTLY SCOPED.
+--
+-- The previous version tested only PAEntryRamp*/PAExitRamp* against buildings
+-- and reported 0/1200 -- but its control ALSO returned 0, which makes the
+-- result vacuous. Worse, the part visibly speared by a tower is a Road_*
+-- segment, i.e. it was sitting in the control group, not the test group.
+--
+-- This version:
+--   * tests EVERY drivable part (Road_*, ramps, PA surfaces) -- 21k parts
+--   * proves the metric can detect a hit BEFORE trusting a zero (positive
+--     control: a synthetic part deliberately placed inside a building)
+--   * reports the worst offenders with coordinates so they can be shot
+
+local ws = game:GetService("Workspace")
+local map = ws:FindFirstChild("Map")
+if not map then
+	return "NO MAP"
+end
+
+local BUILDING = {
+	Building = true,
+	TokyoTower = true,
+	TowerDeck = true,
+	TowerAntenna = true,
+	ShiodomeTower = true,
+	TowerCrown = true,
+	DietTower = true,
+	RoppongiHills = true,
+	TokyoStation = true,
+	StationDome = true,
+	Skytree = true,
+	GinzaBlock = true,
+	NationalDiet = true,
+	BridgeTower = true,
+	TowerCrossbeam = true,
+	PalaceWall = true,
+}
+
+local function isDrivable(n)
+	return n:match("^Road_")
+		or n:find("Ramp")
+		or n == "PAFloor"
+		or n:match("^Apron")
+		or n:match("^Slip")
+		or n:match("^Merge")
+end
+
+local drive, bld = {}, {}
+for _, d in ipairs(map:GetDescendants()) do
+	if d:IsA("BasePart") then
+		if BUILDING[d.Name] then
+			table.insert(bld, d)
+		elseif isDrivable(d.Name) then
+			table.insert(drive, d)
+		end
+	end
+end
+
+if #bld == 0 or #drive == 0 then
+	return ("SCOPE FAIL drive=%d bld=%d"):format(#drive, #bld)
+end
+
+-- Build an OverlapParams limited to building parts only.
+local op = OverlapParams.new()
+op.FilterType = Enum.RaycastFilterType.Include
+op.FilterDescendantsInstances = bld
+op.MaxParts = 12
+
+-- POSITIVE CONTROL: put a probe cube dead-centre inside a building and
+-- confirm the metric reports it. If this returns 0, the metric is broken and
+-- every other number below is meaningless.
+local target = bld[math.floor(#bld / 2)]
+local ctrlHits = ws:GetPartBoundsInBox(CFrame.new(target.Position), Vector3.new(4, 4, 4), op)
+local controlOK = #ctrlHits > 0
+
+local hits, samples = 0, {}
+for _, p in ipairs(drive) do
+	-- shrink slightly so parts merely TOUCHING a facade aren't flagged;
+	-- we want genuine interpenetration
+	local probe = p.Size - Vector3.new(1.5, 1.5, 1.5)
+	if probe.X > 0.5 and probe.Y > 0.5 and probe.Z > 0.5 then
+		local h = ws:GetPartBoundsInBox(p.CFrame, probe, op)
+		if #h > 0 then
+			hits += 1
+			if #samples < 6 then
+				table.insert(
+					samples,
+					("%s@%.0f,%.0f,%.0f in %s"):format(
+						p.Name,
+						p.Position.X,
+						p.Position.Y,
+						p.Position.Z,
+						h[1].Name
+					)
+				)
+			end
+		end
+	end
+end
+
+return ("CONTROL=%s (%d hits on a part inside a building) || drivable=%d bld=%d CLIPPING=%d [%s]"):format(
+	controlOK and "PASS" or "FAIL-METRIC-BROKEN",
+	#ctrlHits,
+	#drive,
+	#bld,
+	hits,
+	table.concat(samples, " ; ")
+)