ADA scanner QA fixture

Purpose-built static fixture with planted, known-count defects (QA_PROPERTY_SPEC.md). Expected findings per page live in HTML comments and EXPECTED.md. Nav mode: server.

• Deliberately clean page
• Planted image-alt defects
• Planted button-name defects
• Planted label defects
• Planted link-name defects
• Planted color-contrast defects
• Severity band edge
• Robots-disallowed canary
• Planted 404 (page does not exist)
• Planted off-host 302