Slate v2 collapsed delete should reuse the before/after location seam

What happened

Once narrow Editor.before(...) and Editor.after(...) were real, the next delete hole was collapsed point/selection deletion with reverse and distance.

The tempting lie was to keep special-casing cursor math inside delete(...).

That would have duplicated location logic in two places and drifted the moment before(...) / after(...) evolved.

What fixed it

The honest follow-on reuses the location seam:

1. resolve the other edge with Editor.before(...) or Editor.after(...)
2. if the result stays in one text leaf, feed that range back through the existing narrow range-delete path
3. if the result crosses into the adjacent mixed-inline sibling leaf, delete the edge text from each side and collapse back to the computed start boundary

That keeps delete(...) small while letting collapsed deletion inherit the same live-draft location rules.

Reusable rule

For Slate v2 text transforms:

• do not duplicate location-walking logic inside delete(...)
• turn collapsed deletion into a range through before(...) / after(...)
• let collapsed mixed-inline delete broaden only as the location seam broadens

If collapsed delete implements its own cursor math, it will rot separately.