Issue dedup tests

Pages designed to exercise the Monitor’s issue deduplication across multiple scan configurations — same project, multiple browsers / viewports / user agents — so that the same logical accessibility issue resolves to a single unique issue with correct per-configuration provenance.

• By configuration — WebKit-only contrast — A contrast violation that appears only when scanned with the WebKit browser engine, used to verify that issue dedup correctly groups instances reported by a subset of a project's scan configurations.
• By viewport — three viewport-gated contrast errors — Three distinct contrast violations, each gated by a different CSS @media viewport-width range (< 300, 330–400, 490–500). Used to verify that issue counts and per-configuration provenance match the configurations whose viewport lands in each gate.