Slate v2 before/after should treat block boundaries as one step

What happened

Once Editor.before(...) and Editor.after(...) could walk across mixed-inline text leaves inside one supported top-level block, the next gap was obvious: they still stopped dead at block boundaries.

That left Transforms.move(...) with a fake ceiling too, even though the model already knew how to represent cursor positions at block ends and block starts.

What fixed it

The honest follow-on treated the boundary between a supported block end and the next supported block start as one step:

1. compute the point offset inside the current block
2. when the offset overflows, carry it into the adjacent supported block with a +1 / -1 boundary adjustment
3. stop only at the document boundary or unsupported blocks

That keeps before(...), after(...), and move(...) aligned on the same cursor-step model.

Reusable rule

For Slate v2 location walking:

• block-end to next-block-start is one step
• previous-block-end to current-block-start is one step
• do not invent an extra fake cursor slot at block boundaries

If block boundaries cost an extra step, cursor motion stops matching the editor model.