[codex] Expose TUS upload chunk size

[tim-kos]

What changed

• Adds tusUploadChunkSize to the Transloadit initializers and passes it through to TUSKit.
• Allows apps to pass 0 to let TUSKit upload each file in one request, which can be useful for iOS background uploads where scheduling a new task for each 500 KiB chunk can stall.
• Clarifies that createAssembly(... andUpload ..., completion:) fires after the Assembly is created and uploads are scheduled, not after file upload or processing completion.
• Documents the recommended lifecycle: use TransloaditFileDelegate.didFinishUpload for upload completion and pollAssemblyStatus for processing completion.
• Adds tests for the default chunk size, a custom chunk size, and the injected-signature initializer.

Why

A customer using TransloaditKit 3.5.0 saw didFinishUpload() fire while API2 had already aborted the Assembly after seeing only three default TUS chunks. Exposing the chunk size gives apps a way to avoid many small background URLSession tasks, and the docs now make the upload lifecycle explicit.

Validation

• swift test