test: add update artwork seams

[TB516]

Summary

• export the artwork driver's fetch, conversion, manifest, sync, and entry-point functions
• guard main() so importing the module is side-effect free
• inject network, command, path, time, and write dependencies at the relevant seams
• cover manifest round-tripping, conversion commands, all collection planners, preflight failure, and the no-change path

Verification

• node --test scripts/update-artwork.test.js
• node --test --experimental-test-coverage scripts/update-artwork.test.js (78.27% line coverage for scripts/update-artwork.mjs)
• npx eslint scripts/update-artwork.mjs scripts/update-artwork.test.js
• npm run lint (0 errors; pre-existing warnings)
• npm test (101/102 files pass; unrelated render-release-card.test.js is blocked by sandbox spawnSync /usr/bin/node EPERM)
• npm run typecheck (blocked by generated JSON seeds absent from a clean checkout)

Fixes #1230

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