Fix iOS autorelease pool lifetime across fiber yields

[tryk016]

The iOS backend currently keeps a local autorelease pool alive while it dispatches an engine event. An event callback can call processEvents() again, yield to UIKit and let the surrounding run-loop pool drain before the engine fiber resumes. Popping the local pool after that can trigger an autorelease-pool badPop crash.

Remove only the pool that spans event dispatch. UIKit still provides the autorelease pool for each event-loop iteration, and the short pool used by the render callback is unchanged.

Validation:

• reproduced the pool corruption with a focused Objective-C++ harness
• 175/175 Tempest tests pass on macOS
• Release builds pass for iPhoneOS arm64 and iPhone Simulator arm64 at iOS 15.0
• an integrated probe on an iPhone 15 Pro Max completes nested processEvents() without a crash