observe
Registers one privileged owner-confined observer without immediately invoking it.
The returned handle is the identity required for geometry publication and must be closed by its retained owner. Closing the current geometry owner permits another viewport to attach while retaining the last metrics snapshot. This runtime extension contract is opt-in and may evolve between minor releases.
Return
privileged observer identity owned by the caller until closed.
Parameters
callback
owner-confined invalidation callback receiving each published metrics snapshot except geometry feedback from its own updateGeometry call; it may read state and release observers but must not write state synchronously.
Throws
when called from another execution owner.