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/push_mapbuilder.py

@@ -0,0 +1,90 @@
+#!/usr/bin/env python3
+"""Push MapBuilder.luau source into a LIVE (Play-mode) Studio via the MCP bridge.
+
+Rojo cannot reach the running server datamodel in Play mode, so the source is
+shipped as ~9000-char chunks into named StringValue children (p0, p1, ...) under
+ReplicatedStorage, then concatenated and assigned to the ModuleScript's .Source.
+
+Named children matter: a retried chunk overwrites its slot instead of appending
+a duplicate. Bridge calls fail intermittently ("Unable to find an active Studio
+instance") even while Studio is up, so every call is retried.
+"""
+import sys, time
+
+sys.path.insert(0, "/tmp")
+import rbx
+
+SRC_PATH = "/home/jesse/roblox/tokyo-wangan-racing/src/server/MapBuilder.luau"
+MODULE = 'game:GetService("ServerScriptService"):FindFirstChild("Server", true)'
+CHUNK = 9000
+
+
+def call(code, tries=4):
+    last = None
+    for a in range(tries):
+        try:
+            r = rbx.call(
+                "execute_luau",
+                {"code": code, "datamodel_type": "Server"},
+                warmup=6,
+                wait=45,
+                timeout=300,
+            )
+            if r and "Unable to find an active Studio" not in str(r):
+                return r
+            last = r
+        except Exception as e:  # noqa: BLE001
+            last = e
+        time.sleep(5)
+    raise RuntimeError(f"bridge failed after {tries} tries: {last}")
+
+
+src = open(SRC_PATH, encoding="utf-8").read()
+nbytes = len(src.encode())
+print(f"source: {len(src)} chars, {nbytes} utf-8 bytes")
+
+# Pick a long-bracket level that does not occur in the source.
+lvl = 0
+while ("]" + "=" * lvl + "]") in src or ("[" + "=" * lvl + "[") in src:
+    lvl += 1
+eq = "=" * lvl
+print(f"long-bracket level: {lvl}")
+
+chunks = [src[i : i + CHUNK] for i in range(0, len(src), CHUNK)]
+print(f"chunks: {len(chunks)}")
+
+print(call('local rs=game:GetService("ReplicatedStorage")\n'
+           'local b=rs:FindFirstChild("MBBuf") if b then b:Destroy() end\n'
+           'b=Instance.new("Folder") b.Name="MBBuf" b.Parent=rs\n'
+           'return "buffer reset"'))
+
+for i, c in enumerate(chunks):
+    code = (
+        'local rs=game:GetService("ReplicatedStorage")\n'
+        'local b=rs:WaitForChild("MBBuf")\n'
+        f'local n="p{i}"\n'
+        "local sv=b:FindFirstChild(n) or Instance.new(\"StringValue\")\n"
+        "sv.Name=n sv.Parent=b\n"
+        f"sv.Value=[{eq}[{c}]{eq}]\n"
+        f'return "p{i} "..#sv.Value'
+    )
+    print(f"  chunk {i+1}/{len(chunks)}: {call(code)}")
+
+assemble = f"""
+local rs=game:GetService("ReplicatedStorage")
+local b=rs:WaitForChild("MBBuf")
+local parts={{}}
+for i=0,{len(chunks)-1} do
+    local sv=b:FindFirstChild("p"..i)
+    if not sv then return "MISSING CHUNK p"..i end
+    parts[#parts+1]=sv.Value
+end
+local s=table.concat(parts)
+local sss=game:GetService("ServerScriptService")
+local m=sss:FindFirstChild("MapBuilder", true)
+if not m then return "MapBuilder ModuleScript NOT FOUND" end
+m.Source=s
+return ("assembled=%d expected={nbytes} match=%s class=%s path=%s"):format(
+    #s, tostring(#s=={nbytes}), m.ClassName, m:GetFullName())
+"""
+print(call(assemble))