Wall the city in and paste posters on it

A 14 m concrete wall now encloses the city 24 m beyond its outermost
blocks. It is in the collision index (not the pick index), blocked on
the navigation grid so residents and routes stay inside, and walk/fly
positions are clamped within it so nobody leaves by flying over.

Posters are pasted on the city side at random spots, sizes and tilts
(up to 25 degrees), one per ~35 m, overlapping like a real hoarding.
They are painted into one atlas from civic slogans plus facts about the
loaded codebase (welcome, tallest tower, largest district, most
connected file), seeded by the repository name so each city's wall is
stable. The wall shader draws concrete courses and joints with a lit
coping, and the minimap outlines the wall. README covers routes,
residents, the wall and security alerts.

README.md

@@ -34,6 +34,11 @@ Configuration:
   - `T` or **Tour** flies over the largest districts. **Copy link** shares the repository, view,
     camera and selected file as a URL fragment (GitHub repositories only).
   - Touch: left thumb joystick, right thumb look, pinch to zoom.
+  - Selecting a file also lays glowing routes along the streets to each file it imports.
+  - Residents wander the streets (toggle in Settings). A wall with posters encloses the city.
+  - Signed in with GitHub, files with open code scanning or Dependabot alerts smoke (and burn
+    for critical/high) and are wrapped in hazard tape. The GitHub App needs read access to
+    code scanning alerts and Dependabot alerts.
 
 Frontend modules (plain ES modules, no build step): `camera.js` (matrices and orbit pose),
 `city.js` (layout, collision, picking, sign placement, tour and links), `labels.js` (text atlas),