Proof of concept StateRegistry

[MiniaczQ]

Objective

Fixes #13711

Solution

Introduce StateRegistry, similar to EventRegistry, which manages state-related system execution
as opposed to the current StateTransitions schedule.
The registry holds a linearized DAG of state dependencies.
The update method applies all transitions and then runs the schedules in 3 waves:

• exits - From leaf to root
• transitions - Arbitrary (implemented as from root to leaf)
• enters - From root to leaf

Pros

• Manual state transition triggered through a StateRegistry::update rather than running a schedule.
• Cleaner ordering, without a lot of after conditions.
• Much simplified custom schedule execution.

Cons

• Potential performance drop.
  While schedule execution is world-exclusive, updates could run in parallel.

Testing

• A single test proves the order is correct.
• Before finalization, this needs benchmarking.

Changelog

TODO

Migration Guide

TODO

[MiniaczQ]

Closed in favor of #13763