Improve frozen mosaic privacy tool

[yvette-carlisle]

Summary

• replace frozen mosaic output with a light privacy texture that does not sample source content
• keep mosaic overlays movable with the pointer while removing mosaic borders
• clear stopped frozen-frame streams so freeze commits rebuild stale authority streams

Validation

• cargo make fmt-swift
• cargo make lint-swift
• git diff --check
• ./scripts/build_and_run.sh run