Fix: renderMediaOnWeb() scroll regression

This PR addresses a regression where renderMediaOnWeb() could make the host page scrollable due to the scaffold node's dimensions.

Why this change?
Previously, the scaffold's fixed element with explicit composition width/height could expand the document.body or documentElement scroll area. This change prevents unintended scrollability of the host page during rendering.

What changed?

1. Scaffold mounting strategy:
   • A new position: fixed; inset: 0; overflow: hidden; visibility: hidden; pointer-events: none; z-index: -9999 wrapper is now appended to document.body.
   • The composition root (the actual rendering target) is mounted as an position: absolute; top: 0; left: 0; child inside this wrapper, with its width/height set from the composition.
2. Cleanup: The cleanup process now correctly removes both the outer wrapper and the inner composition root, in addition to unmounting the React root and cleaning up CSS.
3. Regression Test: A new test in render-media.test.tsx asserts that document.documentElement and document.body scroll dimensions do not increase during or after a renderMediaOnWeb() call.

How it works:
The new fullscreen fixed wrapper effectively isolates the composition's layout from the main document's scroll area. By setting overflow: hidden on the wrapper, any content within (including large compositions) is clipped, preventing it from influencing the host page's scroll dimensions.

Test Evidence:

• packages/web-renderer/src/test/render-media.test.tsx (including the new scroll regression test) passed in Chromium.
• packages/web-renderer/src/test/render-still.test.tsx also passed, verifying shared scaffold path behavior.

Edge Cases/Tradeoffs:

• Preserves existing APIs and backward compatibility.
• Handles compositions larger than the viewport without affecting page scroll.
• Ensures robust cleanup of all DOM nodes.