Add a dual-wheel multi-modal A/B diff engine and visual HTML certification report.

[copybara-service]

Add a dual-wheel multi-modal A/B diff engine and visual HTML certification report.

This change introduces a multi-modal side-by-side (SxS) comparison framework to verify visual rendering, DOM structures, and network traffic across baseline and candidate profiler builds.

Key Capabilities:

1. Multi-Modal A/B Diff Engine (tests/ui/sxs_diff_engine.py):
   • Pixel-by-pixel visual image comparison with configurable color and percentage mismatch thresholds.
   • DOM structural tree diffing with sanitization for dynamic Angular Material and CDK internal attributes to prevent diff noise.
   • Order-independent network request multiset matching with URI query parameter normalization.
   • Content-addressed SHA-256 approval hashing for certifying baseline updates.
2. Self-Contained HTML Certification Report (tests/ui/sxs_report_generator.py, tests/ui/templates/sxs_report_template.html):
   • Standalone HTML certification report with interactive swipe sliders, side-by-side views, and copyable approval tokens.
   • Dynamic template resolver supporting local directories, test runfiles, and package roots.
3. Unit Test Suite (tests/ui/test_sxs_diff_engine.py):
   • Comprehensive unit tests validating visual diff calculations, DOM normalization, manifest approvals, and report generation.
4. Clean Dependencies (tests/ui/requirements-test.txt):
   • Streamlined test requirements.