State management

‍Draft: Verify all lifecycle callback names and transition behavior against current headers.

Purpose

States group related application/game behavior and participate in the engine's update and rendering lifecycle.

Registering and changing states

TODO: Add a tested sample using the public state registration and transition interfaces.

Lifecycle and dispatch

TODO: Explain creation/destruction, input handling, update/render calls, and transition ordering.

Ownership and errors

TODO: Document who owns states and what happens on an invalid transition.

See Architecture.