feat(storage): add StorageTransferTask, MultipartUploadEngine, and upload API

[grdsdev]

Summary

• Adds StorageTransferTask<Success> — a unified handle with AsyncStream<TransferEvent<Success>> progress events and an async value property
• Adds MultipartUploadEngine actor for standard multipart POST uploads with URLSessionTaskDelegate progress events and temp-file streaming for large files
• Merges FileUpload and UploadSource into a single UploadSource type
• Updates upload() and update() to return StorageUploadTask instead of Void

API changes

// upload now returns StorageUploadTask (was Void)
func upload(_ path: String, data: Data, options: FileOptions = .init()) -> StorageUploadTask
func upload(_ path: String, fileURL: URL, options: FileOptions = .init()) -> StorageUploadTask
func update(_ path: String, data: Data, options: FileOptions = .init()) -> StorageUploadTask
func update(_ path: String, fileURL: URL, options: FileOptions = .init()) -> StorageUploadTask

Test plan

• StorageTransferTaskTests — all pass
• MultipartUploadEngineTests — all pass
• StorageFileAPITests — all pass
• UploadProgressTests — all pass

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