feat: add process-scale CLI option

[dofliu]

Summary

• add --process-scale to tune frame scale before MediaPipe processing
• validate scale values as > 0 and <= 1
• use the configured value in the OpenCV resize path
• extend CLI smoke tests and update README/task_progress.md

Validation

• python -m unittest discover -s tests
• python -m py_compile lightsaber_mvp.py

Stack

• stacked on feat: add display-size CLI option #3 (eat/display-size-cli), because feat: add camera-index and no-mirror CLI flags #1-feat: add display-size CLI option #3 are still open.