⭐ Store Devtools

@ngrx/store-devtools — real source, 209/209 tests passing, 0 lint errors. One legitimate DI-token extends reviewed and confirmed clean (a pure type-level anchor, the same category as store's ReducerObservable — see Composition over Inheritance). Surfaced the same store/ActionsSubject composition ripple already seen in effects/router-store, plus a new one specific to this module: a StateObservable-typed usage that needed the same treatment.

Also fixed a real upstream dead-store bug found during the addition — a value written but never read. Not yet covered by the code review audit; queued behind effects.