fix(android): use unstable_getBoundingClientRect for synchronous layout measurement on New Architecture

[wildseansy]

Description

Fixes FlashList rendering nothing on Android New Architecture (RN 0.78+).

Root cause

View.measureLayout() on Android New Architecture invokes its callback asynchronously — it returns before the callback fires. measureLayout() in measureLayout.ts calls it, then immediately returns the local layout object, which is still {x:0, y:0, width:0, height:0}.

This means measureParentSize() always returns {width:0, height:0}, so recyclerViewManager.updateLayoutParams() is called with a window height of 0. FlashList never renders any items.

The onLayout event detects that the stored container size (containerViewSizeRef) is 0 and calls recyclerViewContext.layout() to trigger a re-render — but useLayoutEffect runs measureLayout again with the same async/0 result, creating an infinite re-render loop that never resolves.

Fix

unstable_getBoundingClientRect() reads directly from the Fabric shadow tree and is synchronous on New Architecture (RN 0.78+). Use it in measureLayout when available, and fall back to measureLayoutRelative on Old Architecture where the method doesn't exist.

The commented-out code at the top of measureLayout() pointed at this same API. This PR restores that intent with proper type safety and an Old Arch fallback.

Relationship to #2269

PR #2269 adds an early return when the container measures 0x0 (a different scenario: transient zero measurements during navigation stack transitions on web). Our fix addresses a separate root cause: the measurement itself never returns a non-zero value on Android New Arch. These two fixes are complementary.

Testing

Verified on Android with React Native 0.85 + New Architecture enabled. FlashList renders items correctly after this change. Without it, the list is permanently blank.

Checklist

• Tested on Android New Architecture (RN 0.85)
• Falls back to measureLayoutRelative on Old Architecture
• TypeScript: unstable_getBoundingClientRect is not on the public View type — added a local ViewWithBoundingClientRect type cast