Handle colliders above body if body is becoming taller

ExtendRealityLtd/Zinnia.Unity#157 brought Body Representation that ensures the body is as tall as the user, defined by the headset's position relative to the play area.

If the user stands up while underneath virtual geometry we should not allow the body to increase its height. Instead the body should diverge from the real-world body defined by the headset.

Start by tracing the Body Representation described in ExtendRealityLtd/Zinnia.Unity#157, especially how headset position relative to the play area determines body height. Reproduce the case where the user stands beneath virtual geometry, then identify the collider interaction that should prevent height growth. Done means the body remains constrained under overhead geometry while still following the headset elsewhere.

Written by the indexing model from the issue text.