feat(workflows): add Image Stack block for frame accumulation

[grzegorz-roboflow]

What does this PR do?

New fusion block that accumulates compressed video frames into a
per-camera FIFO buffer, outputting JPEG blobs and frame count.
Designed for shared-hosting safety (always compresses, caps resolution)
and useful for feeding frame sequences to VLMs for action recognition.

Type of Change

• New feature (non-breaking change that adds functionality)

Testing

New tests added

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code where necessary, particularly in hard-to-understand areas
• My changes generate no new warnings or errors
• I have updated the documentation accordingly (if applicable)

Additional Context

N/A