Render C1 structural variety: girder bridges, Ginza overpasses, JCT speeds, 3+3 widening

Four fidelity fixes, all verified in a live Studio datamodel (not just built):

1. Wire up the dead `bridge` flag. Circuit.luau marks 474 samples as bridge
   spans but nothing ever read the field, so every elevated metre rendered as
   the same slab on square columns. Flagged spans now get a steel box girder
   (鋼箱桁橋) with stiffener ribs under the deck, carried on a transverse pier
   cap and round columns at a wider pitch than the viaduct pillars.

2. Ginza 跨道橋. The Ginza section sits below street level in the filled-in
   楓川・築地川 river beds and the surviving city bridges cross over it, their
   piers landing in the median. Adds the crossing decks, parapets and centre
   piers -- which is the physical reason lane changes are banned there, not
   just a paint choice.

3. Junction speed plates. 江戸橋JCT内 / 浜崎橋JCT内 / 京橋JCT-旧白魚橋乗継所 are
   posted 40 km/h, not the 50 km/h mainline limit; plates repeat more often
   through the short restricted stretches.

4. Shiodome JCT -> Hamazakibashi JCT widened to 3+3 lanes via widthAt(), with
   a taper (max ~1.7 studs/sample) instead of a step. Every fixed-width
   consumer -- deck, median, parapets, lane marks, signage offsets, skyline
   setback, finish line -- now reads widthAt() so nothing floats or clips.

Verified headlessly (tools/verify_c1_fixes*.luau):
  girders 488 / 474 flagged samples, pier caps 85, columns 80
  Ginza overpasses 7, piers 7, lateral offset from centreline 0.000 studs
  speed plates 40km/h x7, 50km/h x8
  deck width Shiodome 81.5 vs Kasumigaseki 60.0
  seam lip max 0.0006, road-surface raycast step max 0.260, 0 deck holes

Two earlier "failures" were bad tests, not bad geometry, and the fixed
harnesses document the traps: deck parts are pitched by CFrame.lookAt so
Position.Y+Size.Y/2 is not the surface height (and LookVector is -Z, so the
end faces are easy to swap), and a seam raycast must filter to the Expressway
folder or it hits TunnelCeiling/MedianFence and reports a phantom 19-stud step.

tools/verify_c1_fixes3.luau

@@ -0,0 +1,120 @@
+-- Verification v3: seam lip measured against the KNOWN shared sample point.
+--
+-- v2 still had a sign bug: Roblox CFrame.LookVector is the -Z axis, so a deck
+-- built with CFrame.lookAt(mid, mid+dir) has its FORWARD end at local
+-- z = -(Size.Z-2)/2 and its REAR end at +(Size.Z-2)/2. v2 had these swapped,
+-- comparing deck i's rear to deck i+1's rear -- a systematic error that
+-- reported a near-constant 5.2 on every sloped segment regardless of zone.
+--
+-- This version avoids guessing signs: for each deck it takes BOTH end faces,
+-- picks whichever is nearer the shared sample point b.pos in XZ, and reports
+-- the surface-height difference there. Sanity-checked against the flat-ground
+-- expectation and cross-checked with a downward raycast.
+local sss = game:GetService("ServerScriptService")
+local orig = sss:FindFirstChild("MapBuilder", true)
+local clone = orig:Clone()
+clone.Name = "MapBuilder_Verify3"
+clone.Parent = orig.Parent
+local okM, MB = pcall(require, clone)
+if not okM then
+	clone:Destroy()
+	return "FAIL require: " .. tostring(MB)
+end
+local okB, map = pcall(MB.build)
+if not okB then
+	clone:Destroy()
+	return "FAIL build: " .. tostring(map)
+end
+
+local Circuit = require(game:GetService("ReplicatedStorage"):FindFirstChild("Circuit", true))
+local samples = Circuit.samples
+local n = #samples
+
+local decks = {}
+for _, d in ipairs(map:GetDescendants()) do
+	if d:IsA("BasePart") then
+		local idx = d.Name:match("^Road_(%d+)$")
+		if idx then
+			decks[tonumber(idx)] = d
+		end
+	end
+end
+
+local out = {}
+local function line(s)
+	out[#out + 1] = s
+end
+
+-- Top-surface height of deck `d` at the end nearest world point `p` (XZ).
+local function topAtNearestEnd(d, p)
+	local half = (d.Size.Z - 2) / 2
+	local best, bestD = nil, math.huge
+	for _, z in ipairs({ -half, half }) do
+		local w = d.CFrame * CFrame.new(0, d.Size.Y / 2, z)
+		local dd = (Vector3.new(w.X, 0, w.Z) - Vector3.new(p.X, 0, p.Z)).Magnitude
+		if dd < bestD then
+			best, bestD = w.Y, dd
+		end
+	end
+	return best, bestD
+end
+
+local worst, worstAt, worstGap = 0, -1, 0
+local lips = {}
+for i = 1, n do
+	local j = (i % n) + 1
+	local a, b = decks[i], decks[j]
+	if a and b then
+		local shared = samples[j].pos -- the sample both decks meet at
+		local ya, da = topAtNearestEnd(a, shared)
+		local yb, db = topAtNearestEnd(b, shared)
+		local lip = math.abs(ya - yb)
+		lips[#lips + 1] = { lip, i, math.max(da, db) }
+		if lip > worst then
+			worst, worstAt, worstGap = lip, i, math.max(da, db)
+		end
+	end
+end
+table.sort(lips, function(p, q)
+	return p[1] > q[1]
+end)
+local top = {}
+for k = 1, math.min(5, #lips) do
+	top[#top + 1] = ("i=%d:%.4f(%s)"):format(lips[k][2], lips[k][1], samples[lips[k][2]].piece)
+end
+line(("SEAM max lip=%.4f at i=%d (endpoint XZ err %.3f) -> %s"):format(
+	worst, worstAt, worstGap, worst <= 0.30 and "PASS" or "FAIL"))
+line("SEAM top5: " .. table.concat(top, " "))
+
+-- Independent cross-check: raycast down onto the driving surface just before
+-- and just after each seam. A real step would show up here too.
+-- MUST filter to the Expressway folder only. Filtering on the whole map lets
+-- the ray hit TunnelCeiling / MedianFence / overpass decks on the way down and
+-- reports a bogus ~19-stud "step" that has nothing to do with the road surface.
+local rp = RaycastParams.new()
+rp.FilterType = Enum.RaycastFilterType.Include
+rp.FilterDescendantsInstances = { map:FindFirstChild("Expressway") }
+local rWorst, rAt = 0, -1
+for i = 1, n do
+	local j = (i % n) + 1
+	local p = samples[j].pos
+	local fwd = (samples[(j % n) + 1].pos - samples[i].pos)
+	fwd = Vector3.new(fwd.X, 0, fwd.Z)
+	if fwd.Magnitude > 0.01 then
+		fwd = fwd.Unit
+		local h1 = workspace:Raycast(p - fwd * 3 + Vector3.new(0, 40, 0), Vector3.new(0, -120, 0), rp)
+		local h2 = workspace:Raycast(p + fwd * 3 + Vector3.new(0, 40, 0), Vector3.new(0, -120, 0), rp)
+		if h1 and h2 then
+			local d = math.abs(h1.Position.Y - h2.Position.Y)
+			if d > rWorst then
+				rWorst, rAt = d, i
+			end
+		end
+	end
+end
+line(("RAYCAST surface step across seams (+-3 studs): max=%.4f at i=%d -> %s"):format(
+	rWorst, rAt, rWorst <= 0.60 and "PASS" or "FAIL"))
+
+map:Destroy()
+clone:Destroy()
+return table.concat(out, "\n")