feat: add block windowing support

[srubin]

Summary

Adds opt-in block windowing while keeping the generic lifecycle and DOM protocol inside the Draft fork:

• useDraftEditorBlockWindowing owns measurement, resize invalidation, viewport/overscan calculation, selection-endpoint pinning, and consumer-supplied pinned block keys.
• Stable measurements preserve object identity when ordered block heights have not changed, avoiding a redundant contents render after each edit.
• Structural edits retain measured heights by block key and estimate only new keys from their measured neighbors until the new blocks render.
• Height-only editor resizes no longer invalidate the entire layout; width changes still trigger a full remeasurement because text wrapping can change every block's height.
• DraftEditor replaces omitted runs with inert height spacers while preserving custom block elements and wrapper runs across window seams.
• Offset helpers map both directions needed by consumers: viewport positions to Draft text offsets and Draft text offsets to block keys.

Also bumps the package to 0.11.6-descript.35 for the dependent Descript PR.

Dependent PR: https://github.com/descriptinc/descript/pull/36590

Test plan

• mise exec node@20 -- pnpm exec jest --runInBand --no-watchman src/component/hooks/__tests__/useDraftEditorBlockWindowing-test.ts src/component/contents/__tests__/getDraftEditorWindowedTextOffset-test.ts src/component/contents/__tests__/DraftEditorContents.react-test.tsx (14 tests pass)
• ESLint passes for all changed source and test files.
• mise exec node@20 -- pnpm build completes and emits the new declarations.

The focused tests cover window-range math, stable measurement equality, hook lifecycle and pinning, structural edits with height-only resize notifications, spacer offset mapping, and custom wrapper continuity. Repository-wide declaration checking still reports the fork's existing Node-global and old TypeScript lib-target warnings in unrelated files.