Slate v2 before/after should start as same-text location helpers

What happened

The docs already advertised Editor.before(...) and Editor.after(...), but the v2 core had no real location-walking seam behind them.

The tempting lie was to keep the legacy signatures in docs and quietly guess at cross-node cursor walking later.

That would have created two fake public APIs:

• documented breadth with no proved implementation
• helpers that stop being trustworthy inside active transactions

What fixed it

The first honest cut keeps before(...) and after(...) narrow:

1. support Point and Range
2. for Range, resolve the start edge for before(...) and the end edge for after(...)
3. read the live draft tree
4. walk only within the current text node
5. return undefined at the current text-node boundary

That creates a real location seam without pretending v2 already owns generic cursor walking.

Reusable rule

For Slate v2 location helpers:

• land the smallest honest location seam first
• read the live draft tree, not only committed snapshot state
• add cross-node walking only after the lower-level location APIs are real

If Editor.before(...) or Editor.after(...) promise full location walking before the core can prove it, they are documentation-shaped lies.