Harden invariant evaluation engine and add protocol and packaging contract tests.

[copybara-service]

Harden invariant evaluation engine and add protocol and packaging contract tests.

Key Capabilities:

1. Cross-Realm SVG Geometry & Shadow DOM Invariants (tests/ui/invariants.py):
   • Adds check_svg_geometry_deep() traversing SVG namespaces, Shadow DOM, and iframes via pure Python Playwright locators to detect non-finite numerical coordinates (e.g. NaN attributes in chart trendlines).
   • Adds run_cell_invariants_optimized() with leaf-aligned header inspection, diff column awareness, and poison token scanning ((null), INVALID, [object Object]).
   • Modernizes type annotations to PEP 585 standard collections.
2. Server Protocol & Packaging Integrity (tests/ui/test_server_and_packaging.py):
   • Verifies runtime.js is served with application/javascript MIME type.
   • Verifies unmapped static routes return HTTP 404 rather than being masked by catch-all fallback.
   • Asserts repository working tree purity to prevent server caches from polluting tracked demo files.
   • Verifies proto interface declaration archives (.d.ts.gz) are valid and declared in BUILD.oss.
   • Forbids unexported raw internal import paths in frontend source files.