[ResizeObserver 3/3] ResizeObserver Web API implementation

[paradowstack]

Stack 3/3 — parent: feat/LayoutEventEmitter. Review the two parents first.

Summary:

Implements the ResizeObserver Web API for the New Architecture, behind the enableResizeObserverByDefault flag (off by default).

The motivation is Web compatibility, and it's more capable than onLayout: callers pick which box to observe (content-box, border-box, device-pixel-content-box) and the sizes for those boxes are delivered in the notification.

The design is as follows: JS ResizeObserver/ResizeObserverEntry/ResizeObserverSize on top of a manager singleton and the NativeResizeObserver TurboModule, using the same notify + takeRecords pull model. In C++, ResizeObserverManager collects observed targets whose layout changed at commit time (via shadowTreeDidCommit) and then computes and delivers observations in the event loop's "update the rendering" step (RuntimeSchedulerResizeObserverDelegate::runResizeObservations), as the spec requires. Requires bridgeless + the modern event loop; the legacy scheduler gets a no-op delegate.

Known deviations from the spec (each pinned by a test):

• One gather/broadcast per tick. RN has no synchronous re-layout loop, so the ResizeObserver loop completed with undelivered notifications error is never emitted; a callback that triggers a resize is delivered on the next tick.
• Re-observing a target with the same box is a no-op and does not re-deliver. This matches browsers, not the literal observe() algorithm.
• Callback order across observers follows first-observe() order rather than construction order.

Changelog:

[GENERAL] [ADDED] - Add the ResizeObserver API, behind the enableResizeObserverByDefault feature flag

Test Plan:

• Fantom ResizeObserver-itest.js covers many test scenarios.
• rn-tester has ResizeObserver examples (box sizes, text, visibility); verified initial, resize, and animation-driven delivery there.