Slate v2 Op-family Nineteenth Slice

Supporting plan. For current queue and roadmap truth, see master-roadmap.md.

Goal

• Broaden Editor.before(...) / Editor.after(...) from one text node to the next honest seam: mixed-inline walking inside one supported top-level block.

Scope

• Keep Point | Range support.
• Keep distance.
• Support walking across sibling text leaves inside one supported top-level block, including inline descendants.
• Keep boundary behavior honest:
  • before(...) returns the previous leaf end at a boundary
  • after(...) keeps walking forward through the block text stream
• Avoid cross-block walking and Path support in this slice.

Phases

• Confirm the narrow mixed-inline before/after semantics and current helpers
• Write focused failing tests
• Implement the smallest honest core/API slice
• Sync package/public docs
• Verify the touched package/docs