Implement ProcessPoolExecutor in ComfyStream

[eliteprox]

To provide a reliable API interface for the comfystream BYOC container, we need to separate ComfyUI workflow execution by process

We'll leverage this ProcessPoolExecutor proof-of-concept to execute comfyui prompts in dedicated processes

• pipeline: https://github.com/BuffMcBigHuge/comfystream/blob/e22ceff9a1b9b4a339c7bcea003f99bf6627440f/src/comfystream/pipeline_multi.py
• client: https://github.com/BuffMcBigHuge/comfystream/blob/e22ceff9a1b9b4a339c7bcea003f99bf6627440f/src/comfystream/client_multi.py

Workers will be reused normally and restarted when necessary for unloading cached memory

[linear]

COMFY-118 Implement ProcessPoolExecutor in ComfyStream