Slate v2 Op-family Seventeenth Slice

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

Goal

• Land the first honest location-walking seam: narrow Editor.before(...) and Editor.after(...).

Scope

• Start with same-text location walking only.
• Support Point and Range.
• Support distance.
• Resolve Range edges the honest way:
  • before(...) uses the start edge
  • after(...) uses the end edge
• Return undefined at the current text-node boundary.
• Avoid Path, unit, voids, and cross-node walking in this slice.

Phases

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