Expand description
WitBoundary mirrors for the Effect payload types (plugin-host.md §4.4).
Slice PH7.3b1a: the ~12 nested payload records/enums the effect variant
(PH7.3b1b) composes — Position/Range/Edit/EditDelta/AppliedEdit,
the selection model (Selection/SelectionSet/VisualMode), the modal
model (ModalState/VisualKind/SearchDirection), Register, YankKind.
Each is pure data and its conversion is infallible, but still returns
Result<_, String> to satisfy the uniform WitBoundary contract.
SelectionSet is reconstructed via SelectionSet::from_parts (added to
lattice-protocol for exactly this boundary-projection need — the
counterpart to its all() + primary_index() readers).