Downgrade offscreen Fabric image requests to prefetch (#56976)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56976

changelog: [internal]

Use Fabric layout data to classify image shadow nodes as visible or offscreen on Apple platforms. Offscreen image requests now use `ImageRequestPriority::Prefetch`, which is bridged to the existing `RCTImageLoaderPriorityPrefetch` API, while visible images stay at `Immediate`. The new React Native feature flag defaults to `false` until app-specific gating wires it up.

Reviewed By: javache

Differential Revision: D106074485

fbshipit-source-id: 80ece75cf0d639be1fdbfdd5f19b838b7b64aba6