feat(react): redesign split component

[wangdicoder]

Summary

• redesign Split around a top-level primary pane sizing and collapse model
• refresh the default separator visuals, hit area behavior, tokens, demos, and docs
• add coverage for the new API surface and separator layout behavior

Release

• major: @tiny-design/react
• major: @tiny-design/tokens

Test plan

• pnpm --filter @tiny-design/react test -- split
• pnpm --filter @tiny-design/react exec eslint src/split