Note

Low Risk
Foundation-only change: store and environment injection with no purchase/auth paths touched; behavior is gated behind later wiring of reads/writes.

Overview
Introduces PaywallStateStore, a thread-safe, presentation-scoped holder for declared paywall state keys (defaults, typed writes from stateUpdates, $value payload substitution, reset, and incremental registerDeclarations). It publishes SwiftUI changes via ObservableObject and is exposed as \.paywallStateStore in the environment.

WorkflowPaywallView owns one shared @StateObject store for the whole workflow (empty seed for now) and injects it so state can survive step navigation. PaywallsV2View seeds a local store from stateDeclarations when standalone, but reuses an inherited store when embedded in a workflow so it does not replace the workflow session store.

Unit tests cover seeding, apply/validation/coercion rules, reset, registration, observer notifications, and concurrent access. Wiring stateUpdates and feeding state into ConditionContext are explicitly deferred to later stack items.

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