feat(editor): add offscreen block skeletons

[srubin]

Summary

Adds an Editor-owned block skeleton mode that keeps every Draft block addressable while rendering offscreen contents as raw text instead of the full decorator and leaf tree. Visible, selected, and explicitly pinned blocks continue to render normally, and decorators can opt specific DOM attributes into skeleton output.

This is intended for very large documents where content-visibility avoids paint work but the retained decorator DOM is still expensive. Newly inserted block keys can render directly as skeletons instead of forcing a render-all measurement frame.

Pre-merge follow-up

• Fix intermittent “jump to playhead” behavior when block skeleton mode is enabled, and verify navigation to the playhead before merging.

Test Plan

• NODE_ENV=test jest --no-watchman --runInBand src/component/contents/__tests__/DraftEditorContents.react-test.tsx src/component/base/__tests__/DraftEditor.react-test.tsx — 12 tests passed

• tsc --noEmit --types node,jest,react,react-dom

• eslint src/.

• Vite production library build completed successfully; its declaration plugin continues to report the existing globalThis.__DEV__ diagnostics

• Integrated against the Descript web app with WEB_NODE_ENV=production and REACT_COMPILER=true

• I used AI