Slate v2 ReadOnly Surface Recovery

Goal

Restore readOnly on the structured editing surface so apps do not need to fake the editor root by hand.

Result

• restored readOnly on EditableBlocks / EditableTextBlocks
• widened runtime proof so the structured surface blocks paste/input and exposes the correct read-only hook state
• simplified the read-only example to use the real public surface instead of a hand-built fake editor root

Verification

• yarn workspace slate-react run test
• yarn test:custom
• yarn lint:typescript