fix(iOS): dispatch completion to main thread in handlePreloadedImage

[mfazekas]

Fixes #221

handlePreloadedImage was calling completion() synchronously on the Rive C++ runtime's background import queue, but completion triggers releaseFile() which asserts main thread via dispatchPrecondition. Every other asset path in ReferencedAssetLoader already dispatches to main before calling completion() — this was the only one missing it.

Crash stack (reproduced locally)

_assertionFailure
ReferencedAssetLoader.handlePreloadedImage(_:asset:completion:)
ReferencedAssetLoader.loadAssetInternal(source:asset:factory:completion:)
closure #1 in ReferencedAssetLoader.createCustomLoader(...)
rive::File::import(...)  ← background queue
HybridRiveFileFactory.genericFrom(...)  ← DispatchQueue.global(.userInitiated)

Reproduced using the Out-of-Band Assets (Suspense) exerciser which calls RiveImages.loadFromURLAsync and passes the preloaded RiveImage via referencedAssets.