perf(iOS): avoid blocking a worker during component render

[OskarEichler]

Performance problem and fix

RNNStackPresenter.renderComponents dispatched to a global high-priority queue, dispatched setup back to the main queue, then blocked the global worker with dispatch_group_wait(..., DISPATCH_TIME_FOREVER) until the React top-bar background reported ready.

For waitForRender components, a slow or failed render could occupy that worker indefinitely. The worker and synchronization primitive are unnecessary because readiness is already callback-based.

This change keeps setup asynchronous on the main queue and forwards readiness back to the main queue without blocking any worker. Completion ordering and main-thread delivery remain unchanged.

Regression coverage

The presenter test now exercises the render callback path and verifies completion is delivered on the main thread. The focused presenter suite passes 10/10.

Verification

• New Architecture iOS build-for-testing succeeded
• RNNStackPresenterTest: 10/10 passed on an iPhone 16 Pro simulator (iOS 26.5)
• clang-format and git diff --check passed

Breaking changes

None. This removes internal blocking while preserving asynchronous callback order and thread affinity.