Clarify FixMap website and evidence boundaries

[aryamthecodebreaker]

Summary

• make the homepage explain FixMap through a genuine engine-backed task-to-evidence example
• separate measured retrieval evidence from product mechanism and unmeasured agent outcomes
• strengthen the controlled agent-study protocol without running paid experiments
• correct Product numbering, changelog claim boundaries, and stale sitemap metadata
• demote and tighten the existing video section without replacing its media

Verification

• npm ci
• npm run ci
• npm run lint -w @fixmap/web
• npm run typecheck -w @fixmap/web
• npm run build:web
• desktop and 390px mobile browser checks
• sitemap HTTP/content check

Release boundary

This PR does not deploy, tag, publish, or replace the current video.