Slate v2 Rendering API Recovery

Goal

Recover the remaining honest rendering seams that the docs still expect: renderText, custom placeholder hosts, and leafPosition metadata.

Result

• widened renderLeaf so callers receive: attributes, leaf, text, segment, and leafPosition when the text node is split
• restored renderText on EditableText and threaded it through EditableBlocks / EditableTextBlocks
• restored custom placeholder host rendering with renderPlaceholder
• widened runtime proof for: custom text hosts, custom placeholder hosts, leafPosition, and top-level forwarding through EditableBlocks
• aligned the rendering docs to the recovered surface

Verification

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