Refactor nested layout

Problem/Motivation

Entity tempstore swapping (loading the tempstore version of an entity during page rendering) was implemented in edit_plus's EntityConverter param converter, which hardcoded permission and mode checks. This meant any module wanting tempstore entity swapping had to depend on edit_plus, and there was no way to layer activation logic from multiple modules.

Proposed resolution

• Introduce a TempstoreActivationCheckerInterface with isActive() and onEntitySwapped() methods, allowing modules to decorate the service to layer their own activation logic (e.g., navigation_plus checks edit mode, edit_plus checks permissions).
• Provide a DefaultTempstoreActivationChecker (always inactive) so tempstore_plus works standalone without swapping anything until a decorator opts in.
• Move the EntityConverter param converter decorator from edit_plus into tempstore_plus, wired to the activation checker.

API changes

• New TempstoreActivationCheckerInterface — modules decorate tempstore_plus.activation_checker to control when tempstore entity swapping is active.
• New tempstore_plus.param_converter.entity service — decorates core's paramconverter.entity.