Fix ScrollBox scissor clipping for nested scrollboxes

[acoliver]

Fixes #388.

Summary

• intersect scissor rectangles when nesting ScrollBoxes to prevent children from drawing outside parents
• remove extra padding on ScrollBox viewport to avoid bleed
• add core and React regression tests covering nested scrollboxes and stacked tool blocks

Testing

• bun test packages/core/src/tests/scrollbox.test.ts
• bun test packages/react/tests/layout.test.tsx

[acoliver]

before:

after:

you'll also find a test.