A12Path
One part of an exact A12 pointer — the counterpart of A12's PathPart: a name and a repetitionIndex.
The domain is PathPartImpl's, and it is looser than the pointer STRING grammar. A non-empty name and repetitionIndex >= 0 ("repetition index must be > 0, or 0 for wildcard, but not negative") — and nothing else. PathPart.of("a/b", 1) and PathPart.of("Order$", 1) are both accepted by the kernel and survive into DocumentPointer.fullName(); only DocumentPointer.of(String) refuses those names, because a written pointer has to parse back to the same value. This type previously imposed the string grammar here, which made a legal A12 part unrepresentable — an over-rejection on the read path as much as the authoring one.
Whether a 0 may sit at this position is the pointer's rule, not the part's, exactly as in A12: DocumentPointerImpl's constructor decides that, and its assertion records that "PathPart is responsible for repetition indexes >= 0".
A data class is safe here where it is not on A12Pointer: this holds no collection, so copy() re-runs the primary constructor and re-validates rather than side-stepping a snapshot.