Is valid checks if anything exists at a path after composition instead of if an authored spec of the correct kind exists.

[NicTanghe]

I think this changes what thr function is supposed to be used for.
THe c++ impl reads.

bool IsValid() const {
    if (!UsdIsConcrete(_type) || !_prim)
        return false;

    if (_type == UsdTypePrim)
        return true;

    SdfSpecType specType = _GetDefiningSpecType();

    return (_type == UsdTypeAttribute &&
            specType == SdfSpecTypeAttribute) ||
           (_type == UsdTypeRelationship &&
            specType == SdfSpecTypeRelationship);
}

1. The object must be concrete (not abstract / schema-only)
2. There must be a defining authored spec
3. If it’s a Prim → valid immediately
4. If it’s an Attribute → defining spec must be Attribute
5. If it’s a Relationship → defining spec must be Relationship

Current

self.stage.prim_index(&self.path).is_some()

seems to not check spec.