Extract linear-layout extension from tensor-viz

[latentCall145]

Summary

• make the viewer demo shell accept extension factories at startup
• publish a side-effect-light @tensor-viz/viewer-demo/extension-api surface for downstream extensions
• remove the linear-layout extension and generated-example sync hook from tensor-viz
• make viewer-demo packageable as a library while preserving the standalone demo build

Verification

• npm run check:ts-docs
• npm run test
• npm run build
• npm pack --workspace @tensor-viz/viewer-core
• npm pack --workspace @tensor-viz/viewer-demo