Deprecate DisconnectedSpace, replace with pure transform marker type InvalidTransform/UndefinedTransform (tbd)

[Wumpf]

While sitting over

• DisconnectedSpace snippet is broken #6817

@jleibs and me realized that given how the rest of the Viewer evolved the concept of a "disconnected" space no longer makes sense: it's not trivial to configure entity path expressions to achieve what we previously tried to achieve with DisconnectedSpace

We still want a pure spatial (not topological like DisconnectedSpace) property that disonnects the transform tree itself. This would not have any effect on either

• internal topology
• heuristics
• visualizability

I.e. this preserves what we handle within the transform context as UnreachableTransformReason::DisconnectedSpace, but removes all other aspects of disconnected space.

---

We should introduce this new archetype and deprecated DisconnectedSpace in the same release, making it equivalent to the new type as long as it is shipped under a deprecation mark.