sysdesign: iPad authoring client for the SF Building Template Pipeline

[nigel-hall-codes]

The PDF's primary authoring tool — too large for a leaf, so it gets its own design phase. Browse buildings, load footprints/templates, 3D view, import screenshots, trace facade elements, author parts (GLB) + material roles, request AI signs, author placement rules, save to the server. Talks only to the Home PC Server (never AI or Unity directly).

Depends on the server contract (data-model.md §5).

Output: sdlc/#<this>/design.md. From parent design #266 (sysdesign).

[nigel-hall-codes]

Coordination from #278 (building-specific facade canvas, sysdesign done):

The facade canvas is designed as a mode inside this iPad client, not a separate app. When drafting #276's design, please reserve a slot for it:

• a building picker → facade picker (using the bake sidecar's ranked street_facades[] + non-street faces)
• a 2D canvas surface over the chosen facade's unit square (x,y ∈ [0,1], the same facade frame templates use)
• freehand strokes + placed images / AI signs (feat(server): AI sign generation - provider abstraction #275) as reorderable layers, saved to the server (feat(server): canvas storage + flatten-on-export to facadeDecals override #281)

Implementation issue: #282 (blocked-by this issue + #281). Design detail: sdlc/#278/design.md in the #278 worktree. Goal is to keep the two designs from diverging on the building/facade picker.

[nigel-hall-codes]

sysdesign delivered: sdlc/#276/design.md on branch sdlc/#276-sysdesign — iPad authoring-client architecture (server-only boundary, component map, client↔server contract on the now-built §5 endpoints, facade-frame inheritance, sync model, the #278 canvas mode, decisions, contract gaps, MVP slice). Implementation is the already-captured #282 (facade canvas mode) + successors; not auto-planning further children to avoid scope ballooning.