fix(street): taper sidewalk upstand to flush at the building edge

The sidewalk stood the full 0.15 m kerb height above the DEM at *both* edges, so
the outer (building-side) lip was a 15.8 cm ledge where the walk route crossed
from Terrain_DEM onto Sidewalk_South. sidewalk_z now tapers the upstand from full
at the road-side edge to zero at the building-side edge (a ~3-4 % ramp, walkable
and visually negligible).

Pre-check: south max step 20.5 -> 13.4 cm, north 15.1 -> 7.2 cm (the residual
13.4 cm is the crossing-ramp gradient, tracked separately). Street and site
rebuilt, site re-imported (7/7 meshes, collision flags intact), DemoMap rebuilt;
packaged -qaselftest still PASS. README step table updated.

Docs/Architecture/architecture.md

@@ -1,8 +1,9 @@
-# Gig Line Architecture Documentation
-
-> Linux-first walking/driving demo around 6840 Hollywood Blvd
-
-Version 0.1.0 — 2026-11-08 — initial architecture documentation
+---
+title: Gig Line Architecture Documentation
+description: Architecture documentation for the Gig Line Linux-first Unreal Engine 5 demo around 6840 Hollywood Blvd.
+status: v0.1.0
+date: 2026-11-08
+---
 
 # Introduction and Goals