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.

docs/japanese-road-standards.md

@@ -43,7 +43,7 @@ blue. Shutoko uses green.
 
 | Route | Posted |
 |---|---|
-| C1 都心環状線 (Inner Circular) | **50 km/h**, dropping to **40 km/h** in the tightest junction sections (e.g. Edobashi JCT) |
+| C1 都心環状線 (Inner Circular) | **50 km/h** mainline, **40 km/h** inside the junction sections — 江戸橋JCT内, 浜崎橋JCT内, and 京橋JCT–旧白魚橋乗継所 are each posted 40 in the route's own 車線・最高速度 table |
 | B 湾岸線 (Bayshore/Wangan) | **80 km/h** (60–70 in places) |
 
 The C1 and the Wangan are **not** the same limit — a C1-shaped circuit should be
@@ -58,6 +58,36 @@ posted 50, not 80.
   photometry. Porous asphalt (排水性舗装) does read darker and coarser than
   ordinary asphalt, which the palette reflects.
 
+## Structural sections of the C1 (drives MapBuilder geometry)
+
+The C1 is not a uniform elevated tube — it changes structural type along its
+length, and the circuit now renders each type differently.
+
+| Section | Real-world form | In-game |
+|---|---|---|
+| Elevated viaduct (most of the loop) | Deck on concrete columns | `Pillar` every 3rd sample above Y=18 |
+| Bridge spans (474 flagged samples) | Steel box girder 鋼箱桁橋 over the old canals | `BridgeGirder` + `GirderRib` under deck; `PierCap` + round `PierColumn` on a 6-sample pitch |
+| Ginza 銀座 | Runs **below** street level in the filled-in 楓川・築地川 river beds; surviving city bridges cross **over** it, piers landing in the median | `GinzaOverpass` + `OverpassParapet`, `OverpassPier` in the median |
+| Tunnel sections | Bored/cut-and-cover | Tunnel bore, ceiling, no sky lighting |
+
+Girder depth is set from the span/depth rule of thumb for steel box girders
+(≈1/20 of span): at ~35 m spans that is ~1.8 m ≈ 6.4 studs (`GIRDER_H`).
+
+**Why Ginza bans lane changes.** The no-lane-change marking through Ginza is not
+arbitrary styling — the crossing bridges' piers physically occupy the median
+between the carriageways, so the lanes are separated by structure. The yellow
+line and the `OverpassPier` geometry are two expressions of the same fact.
+
+## Lane counts
+
+| Section | Lanes |
+|---|---|
+| Most of C1 | 2 + 2 |
+| Shiodome JCT → Hamazakibashi JCT | **3 + 3** (widened; carries the merge from the Haneda line) |
+
+Rendered by `widthAt(i)`: 60 studs baseline, 82 studs in `WIDE_ZONES`, with a
+taper between so the widening never steps more than ~1.7 studs per sample.
+
 ## Method note
 
 Wikipedia article *extracts* (`prop=extracts`) silently drop the infobox and