fix(ios): dispatch Dictionary access to main thread in offline controllers

[mateusztylman]

Summary

Fix thread-unsafe Dictionary access in TileStoreController.swift and OfflineController.swift that causes EXC_BAD_ACCESS (SIGSEGV) crashes when downloading tile regions or style packs.

Fixes #1116

Problem

loadTileRegion, estimateTileRegion, and loadStylePack use progress and completion closures that are called on MapboxCommon background threads. Both closures access shared handler dictionaries without synchronization:

• Progress callbacks: read from the dictionary on a background thread
• Completion callbacks: removeValue(forKey:) runs on a background thread (executeOnMainThread only wrapped the completion call, not the removeValue that followed)

Swift's Dictionary is not thread-safe for concurrent read+write. When multiple regions/packs have overlapping lifetimes, this causes crashes.

Fix

Wrap both progress and completion closure bodies in DispatchQueue.main.async so all dictionary access is serialized on the main thread. This also ensures Flutter event sink calls happen on the main thread, consistent with Flutter's platform channel contract.

Files changed:

• TileStoreController.swift - loadTileRegion and estimateTileRegion
• OfflineController.swift - loadStylePack

Crash signatures (from production)

// Crash 1 - concurrent read during mutation
Thread 29: specialized __RawDictionaryStorage.find<A>(_:)
           specialized Dictionary.removeValue(forKey:) (TileStoreController.swift:33)

// Crash 2 - CoW uniqueness check on freed backing storage
Thread 23: swift_isUniquelyReferenced_nonNull_native
           specialized Dictionary.removeValue(forKey:) (TileStoreController.swift:33)

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