Slate v2 Focus History Policy Cleanup

Goal

Make the current focus/history policy beta-clean:

• History control hooks name DOM focus behavior explicitly.
• Root chrome names selection placement explicitly.
• Root chrome restore restores the last known selection for that root.
• Existing mounted-root editor focus behavior stays intact.

Plan

1. Add regression coverage for root chrome restoring an inactive root caret.
2. Rename public options:
   • useSlateHistory({ focusPolicy })
   • useSlateRootChrome(root, { selection })
3. Store last non-null selection per root in Slate React runtime context.
4. Use mounted root editors for DOM focus, and last per-root selection for chrome restore.
5. Run focused React tests, typechecks, lint, and browser example proof.

Notes

• docs/solutions/patterns/critical-patterns.md is absent in this checkout.
• Relevant prior learnings:
  • docs/solutions/ui-bugs/2026-05-23-slate-react-mounted-root-editor-focus-for-root-chrome-history.md
  • docs/solutions/ui-bugs/2026-05-21-slate-v2-multi-root-chrome-clicks-must-activate-root-before-focus.md

Result

• useSlateHistory uses focusPolicy for DOM focus behavior.
• useSlateRootChrome uses selection for root caret placement.
• Runtime context stores last known selection per root.
• Root chrome restores inactive root carets and leaves already-focused editable clicks native.
• History and root chrome share focusSlateEditable, which focuses the mounted editable element before asking Slate DOM focus to sync selection.

Verification

• bun --filter slate-react test:vitest -- ./test/use-slate-root-chrome.test.tsx ./test/use-slate-history.test.tsx
• bun --filter slate-react typecheck
• bun typecheck:site
• bun lint:fix
• bun build:next
• PLAYWRIGHT_BASE_URL=http://localhost:3123 PLAYWRIGHT_RETRIES=0 bun run playwright playwright/integration/examples/multi-root-document.test.ts --project=chromium --workers=1