Daikoku PA: two perimeter ramps, far side served by the median opening

Rework the PA junction to match the reference drawing: yellow = PA -> highway
(entry), red = highway -> PA (exit). Entry and exit swap sides (entry now
EAST, exit WEST) and both trunks hug the PA perimeter instead of running
concentric spirals.

The simplification: both carriageways sit at the same deck level, adjacent,
so the median opening IS the far-side access path. The under-deck crossing
and merge runs are deleted outright -- 8 runs collapse to 4, and the
girder-soffit ceiling logic that constrained the crossing goes with them.
Part count 24,210 -> 21,691.

The median opening is now load-bearing rather than cosmetic (it is the only
route from the far carriageway to the ramps), so it widens from 6 to 9
samples per junction.

Orientation is a single flip-able constant (ORIENT) so a mirrored result is
a one-line correction, not a rewrite.

Measured on the built map (not asserted):
  - 600 entry + 600 exit ramp parts
  - max grade 8.00% / 8.27%, under the 9% limit
  - 0 collisions with piers/masts/girders (558 structural parts)
  - segment continuity <= 2.5 studs; trunk->merge handoff <= 1.7 studs
  - merge lands at dY 0.0 (exact deck level), 33 studs lateral
  - median open at junctions: 0 parts vs. control ~7 on plain mainline

Two traps worth recording, both caught by controls:

1. Studio's require() cache silently rebuilt STALE code. The first "passing"
   build reported 17,575 parts and zero ramps, and the map contained
   PAOffRamp/PAOnRamp parts whose names exist nowhere in the current source.
   Every measurement taken before this was describing an old map. Added
   tools/rebuild_fresh.luau, which clones the module to bust the cache; use
   it rather than requiring MapBuilder directly.

2. A reported "median never opens" bug was a bad probe, not a defect. It
   counted `Barrier` (the outer edge parapet, which must NEVER open) instead
   of `Median_<i>`. Re-run against the right name: control 7, junctions 0.

Both were only visible because the probes carry controls -- a 0-hit result
with no control is vacuous. The verification harnesses are kept in tools/
with their controls built in.

tools/measure_new_ramps.luau

@@ -0,0 +1,110 @@
+-- Measure the redesigned ramps: extent, grade, side (E/W), and collisions
+-- with STRUCTURAL parts (piers, masts, girders). Read-only.
+local ws = game:GetService("Workspace")
+local map = ws:FindFirstChild("Map")
+if not map then
+	return "no Map"
+end
+
+local PA = Vector3.new(-4124, 46, -7470)
+
+-- Gather deck parts per run.
+local runs = {}
+for _, d in ipairs(map:GetDescendants()) do
+	if d:IsA("BasePart") then
+		local nm = d.Name
+		for _, base in ipairs({ "PAEntryRampTrunk", "PAEntryRampNear", "PAExitRampTrunk", "PAExitRampNear" }) do
+			if nm:sub(1, #base) == base then
+				runs[base] = runs[base] or {}
+				table.insert(runs[base], d)
+			end
+		end
+	end
+end
+
+local out = {}
+local order = { "PAEntryRampTrunk", "PAEntryRampNear", "PAExitRampTrunk", "PAExitRampNear" }
+for _, base in ipairs(order) do
+	local parts = runs[base]
+	if not parts then
+		table.insert(out, base .. ": MISSING")
+	else
+		local minY, maxY = math.huge, -math.huge
+		local minX, maxX = math.huge, -math.huge
+		local minZ, maxZ = math.huge, -math.huge
+		local maxGrade = 0
+		table.sort(parts, function(a, b)
+			return a.Name < b.Name
+		end)
+		for _, p in ipairs(parts) do
+			local q = p.Position
+			minY, maxY = math.min(minY, q.Y), math.max(maxY, q.Y)
+			minX, maxX = math.min(minX, q.X), math.max(maxX, q.X)
+			minZ, maxZ = math.min(minZ, q.Z), math.max(maxZ, q.Z)
+		end
+		for i = 2, #parts do
+			local a, b = parts[i - 1].Position, parts[i].Position
+			local horiz = (Vector3.new(a.X, 0, a.Z) - Vector3.new(b.X, 0, b.Z)).Magnitude
+			if horiz > 0.5 then
+				maxGrade = math.max(maxGrade, math.abs(b.Y - a.Y) / horiz * 100)
+			end
+		end
+		-- mean X relative to PA centre tells us east (+) or west (-)
+		local sx = 0
+		for _, p in ipairs(parts) do
+			sx += p.Position.X
+		end
+		local side = (sx / #parts) > PA.X and "EAST" or "WEST"
+		table.insert(
+			out,
+			("%s: n=%d Y=%.0f..%.0f grade<=%.2f%% side=%s X=%.0f..%.0f Z=%.0f..%.0f"):format(
+				base,
+				#parts,
+				minY,
+				maxY,
+				maxGrade,
+				side,
+				minX,
+				maxX,
+				minZ,
+				maxZ
+			)
+		)
+	end
+end
+
+-- Structural collision test: does any ramp deck part overlap a pier/mast?
+local structural = {}
+for _, d in ipairs(map:GetDescendants()) do
+	if d:IsA("BasePart") then
+		local nm = d.Name
+		if nm:find("PierColumn") or nm:find("PAMast") or nm:find("GirderRib") then
+			table.insert(structural, d)
+		end
+	end
+end
+table.insert(out, "structuralParts=" .. #structural)
+
+local hits = 0
+local detail = {}
+for _, base in ipairs(order) do
+	local parts = runs[base] or {}
+	local h = 0
+	for _, p in ipairs(parts) do
+		for _, s in ipairs(structural) do
+			local dx = math.abs(p.Position.X - s.Position.X)
+			local dz = math.abs(p.Position.Z - s.Position.Z)
+			local dy = math.abs(p.Position.Y - s.Position.Y)
+			local rx = (p.Size.X + s.Size.X) / 2
+			local rz = (p.Size.Z + s.Size.Z) / 2
+			local ry = (p.Size.Y + s.Size.Y) / 2
+			if dx < rx and dz < rz and dy < ry then
+				h += 1
+			end
+		end
+	end
+	hits += h
+	table.insert(detail, base .. "=" .. h)
+end
+table.insert(out, "STRUCTURAL_HITS total=" .. hits .. " (" .. table.concat(detail, " ") .. ")")
+return table.concat(out, "\n")