Parsed URL. For HTML-relative URLs, resolved against https://observer.test/.
Attribute the URL was extracted from (href, src, srcset, data-impression-url, ...).
HTML tag the URL was extracted from (a, img, iframe, meta, ...).
Best-effort line number in the preview HTML, 1-indexed. null if unknown.
Shared types for the substitution observer/encoder surfaces. The shapes match the test-kit contract at
compliance/cache/latest/test-kits/substitution-observer-runner.yaml.