Split area build orchestration implementation plan

Extract vehicle route helpers into vehicle-route.js; add unit coverage using fixture OSM.
Extract vehicle glTF builder into vehicle-model.js; assert schema, mesh/material counts and data URI.
Extract preview support-file copy, HTML template, escaping and summary into area-preview.js.
Replace build-area.js bodies with imports/calls while preserving preview manifest ownership and ordering.
Run Node syntax/unit tests, then only the nantaizi preview, diagnose and quality gate commands.
Update pipeline/preview specs and changelog; inspect diff for accidental stage or output changes.

Validation

node --check scripts/build-area.js
node --check scripts/lib/vehicle-route.js
node --check scripts/lib/vehicle-model.js
node --check scripts/lib/area-preview.js
npm run test:preview-assets
npm run build:area -- --config config/areas/nantaizi-lake-innovation-valley.json --stages preview
npm run diagnose:area -- --config config/areas/nantaizi-lake-innovation-valley.json
npm run check:area -- --config config/areas/nantaizi-lake-innovation-valley.json
git diff --check