Fix package-root tree shaking

[xuewill]

Summary

• declare @webaseui/svelte side-effect free so unused components and styles are tree-shaken
• add package-contract and packed-consumer regressions for unused component code
• prepare patch release 0.3.1

Verification

• npm run check
• npm run check:consumer
• npm audit --audit-level=high
• xue performance budget: ordinary routes return below their existing JS/CSS limits