Expose scene transform builder

[agustin-littlehat]

Summary

• add public buildPolySceneTransform / PolySceneTransformInput in core
• re-export the scene transform helper and transform input types from vanilla, React, and Vue entrypoints
• route vanilla, React, and Vue camera scene-root transform construction through the shared helper
• document the helper in the README and headless API docs

Why this helps

This gives external render adapters a supported way to build the same scene-root transform string PolyCSS uses internally, including target + auto-center offset conversion, zoom normalization, optional distance, layout scale, and world-unit sizing. It also removes duplicated transform string math from the framework camera paths.

Validation

• pnpm --filter @layoutit/polycss-core test -- src/transform/sceneTransform.test.ts
• pnpm --filter @layoutit/polycss test -- src/api/scene/transforms.test.ts src/index.test.ts
• pnpm --filter @layoutit/polycss-react test -- src/camera/useCamera.ts
• pnpm --filter @layoutit/polycss-vue test -- src/camera/useCamera.ts
• git diff --check
• pnpm build:packages
• declaration output contains buildPolySceneTransform and PolySceneTransformInput for core, vanilla, React, and Vue
• pnpm test && pnpm build

Note: the existing happy-dom iframe abort/network stderr and React act(...) warnings still appear during tests, but all tests pass.

Supersedes closed PR #67, which used the wrong temporary branch name.