chore(task): archive 08-04-split-area-build-orchestration

.trellis/tasks/archive/2026-08/08-04-split-area-build-orchestration/design.md

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+# Split area build orchestration design
+
+## Module Boundaries
+
+```text
+scripts/lib/vehicle-route.js   OSM XML -> preview route object
+scripts/lib/vehicle-model.js   -> vehicle glTF object
+scripts/lib/area-preview.js    HTML/template + runtime file copy + preview summary
+scripts/build-area.js          dependency checks, writes, stage timing, manifest ownership
+```
+
+`vehicle-route.js` and `vehicle-model.js` are pure Node modules. `area-preview.js` may use Node filesystem and path, but receives all names/paths through arguments; it must not import area config, stage manifest or spawn processes.
+
+## Preview Flow
+
+`build-area.writeCesiumPreview()` continues to ensure GLB/metadata, establish timing, write route/model/runtime/HTML in the existing order, and write `preview.manifest.json`. It delegates route construction, vehicle construction, support copy and HTML generation to the three modules.
+
+## Compatibility
+
+Public CLI and all generated filenames remain unchanged. The route's `generatedAt` naturally changes on each generation and is not a stable parity field; route segments, vehicle glTF structure and preview config are the stable contract. No Blender/GLB parity run is needed because no Blender or GLB generation code changes.
+
+## Rollback
+
+Restore the extracted functions to `build-area.js` and delete the three modules; no data migration or output cleanup is required.