Slate v2 delete should start with exact path support only

What happened

The docs already exposed Transforms.delete(...), but the package had no helper at all.

The tempting lie was to jump straight to broad legacy delete semantics:

• point deletion
• range deletion
• unit/distance
• reverse
• hanging

That would have been more API than engine.

What fixed it

The honest first cut stayed tiny:

• exact { at: Path } support only
• reuse the existing remove_node seam
• work correctly inside an outer transaction against the live draft tree

Reusable rule

For broad transform families in Slate v2:

• start from the exact case the engine already owns
• do not ship the whole legacy option bag before the semantics are real

If the helper surface is broader than the underlying engine seam, it is fake progress.