Epic 106: add video recognition substrate

[rsasaki0109]

Summary

• add deterministic dense block flow, adaptive background modeling, and IoU multi-object tracking
• isolate timestamped stream adapters behind additive video features
• expose dense flow to Python and add OpenCV Farneback comparison receipts
• document architecture, API stability, and Epic 106 completion

Validation

• cargo test -p spatialrust-vision --no-default-features --features video-adapters
• cargo test -p spatialrust-vision --features full
• cargo clippy -p spatialrust-vision --no-default-features --features video-adapters -- -D warnings
• cargo test -p spatialrust-platform
• cargo bench -p spatialrust-vision --features video --bench video --no-run
• pytest crates/spatialrust-py/tests
• python bench/opencv_comparison/run.py --suite video
• python -m unittest test_report.py