Slate v2 mixed-inline range delete should share the edge-delete helper

What happened

Once collapsed mixed-inline delete across adjacent sibling leaves was real, the next obvious hole was explicit non-empty Range deletion for the same shape, and then the wider version where the range fully covers interior descendants between the start/end edges.

The tempting lie was to bolt a second bespoke mixed-inline delete path onto explicit ranges and leave collapsed delete on its own helper.

That would have created two code paths for the same structural shape.

What fixed it

The honest follow-on reused the same edge-delete helper as the entry point and then let it grow to remove fully covered interior descendants too, for both:

1. explicit non-empty Range
2. current non-empty selection when at is omitted

So mixed-inline range delete now has one structural implementation regardless of whether the range started collapsed and was expanded through before(...) / after(...) or was already explicit, and regardless of whether the interior span is only adjacent sibling leaves or also contains fully covered descendants.

Reusable rule

For Slate v2 delete semantics:

• if two delete shapes collapse to the same structural leaf-boundary case, they should share one helper
• do not fork collapsed and explicit range deletion unless the tree shape truly differs

If mixed-inline range delete needs two helpers for the same shape, one of them is probably bullshit.