Slate v2 collapse should read the live draft selection

What happened

After setSelection(...) and deselect(...), the next obvious helper was collapse(...).

The trap was the same as before:

• read editor.selection
• collapse that value
• call it done

That would have been wrong inside an outer transaction, because editor.selection still points at committed state while the draft selection may already have changed.

What fixed it

The honest helper reads the same seam as the other narrow selection helpers:

• draft selection when a transaction is open
• committed snapshot selection otherwise

Then it collapses to one of four edges:

• anchor
• focus
• start
• end

Reusable rule

For Slate v2 selection helpers:

• edge semantics are easy
• draft-state correctness is the real problem

If collapse(...) reasons from committed selection while draft edits are still in flight, the helper is not transaction-aware no matter how nice the API looks.