Document city traffic; refresh street screenshots

docs/ARCHITECTURE.md

@@ -80,7 +80,9 @@ Plain ES modules, no framework, no build step.
 3. **Doors and street furniture:** small instanced boxes (lit doorways, lamp posts and heads, bins,
    benches, trees, hydrants, post boxes) drawn with a separate shader variant, and only within a few
    hundred metres of the camera. Streetlights stand on folder boundaries, and everything sits on
-   folder pavements, never the road.
+   folder pavements, never the road. Cars use the same shader: each block inside a folder has a
+   one-lane loop half-way into the street around it, and only cars near the camera are rebuilt
+   each frame.
 4. **Effects:** additive and never occluding. Lamp light pools, search beacons, fire and smoke particles (stateless,
    animated in the vertex shader), residents (procedural stick-figure sprites), street routes
    (mitred ribbons with chevrons or pulsing dashes, kept in right-hand lanes) and import arcs.

docs/CITY.md

@@ -24,6 +24,7 @@ scale, so a large repository is a large city.
 | **Lit doorway** | The building's door: facing the street, or a back door onto an alley for files in the middle of a folder |
 | **Streetlights** with pools of light | The edges of each folder: lights stand on its boundary, at every corner and every 20 m or so |
 | **Bins, benches, trees, hydrants, post boxes** | Street furniture on the folder's pavement. Purely for atmosphere |
+| **Cars** | Traffic circling the blocks inside each folder, on its grey streets (never on the black ground between districts). They drive on the right and stop for you if you stand in their lane. Purely for atmosphere |
 | **Cyan chevrons** on the road | Route to a file the selected file imports (**amber** if the import was inferred) |
 | **Pulsing magenta dashes** on the road | Route from a file that imports the selected file |
 | **Arcs** in the sky | The same import relationships, as the crow flies |