feat(python): emit per-facade edge geometry in the building sidecar

[nigel-hall-codes]

Parent: #278 (building-specific facade canvas) — see sdlc/#278/design.md analyze finding A2.

Extend the per-chunk building sidecar (chunk_CC_RR_buildings.json, owned by #268) so each entry carries, per facade, the wall edge's world-space geometry the Unity decal importer needs to anchor a quad:

• edge endpoint coordinates in world meters (the two footprint vertices of that facade edge)
• base elevation (terrain-adapted base height) + facade height (floor_count * floorHeight)
• keep the existing street_facades[].edge_index so a facade is identified by stable edge identity (analyze A5)

Why: buildings are merged into one combined mesh at import, so per-building transform is gone by decal-placement time. The sidecar must hand the importer a world-space anchor for each facade directly.

Blocked-by #268 (extends that sidecar). No Unity changes here.