Opting into React's experimental channel — which emits <link rel="expect"> to hold first paint until the streamed shell is coherent, avoiding flicker from partially-streamed HTML — currently requires enabling an unrelated feature like experimental.taint as a side effect, which is confusing. This adds experimental.useExperimentalReact as a direct opt-in.

It feeds the existing needsExperimentalReact aggregation and the matching Turbopack react_channel switch, selecting the react@experimental build the same way taint, transitionIndicator, and gestureTransition do. It's opt-in only: an explicit false can't disable the channel when one of those still requires it (the taint APIs only exist in the experimental build), so assignDefaults warns on that contradiction. Covered by a webpack e2e test mirroring the existing taint channel test.