feat(site): agentdiff-style 404 report with typed terminal diff - #21

What

Custom 404 that speaks fluent AgentDiff: the visitor's requested URL is diffed against the sitemap and fails every gate (TDI 1.000, refresh-loop ×3, RSR ∞) in a typewriter terminal report — with their URL pinned as the culprit. Mobile gets a compact verdict card instead of the terminal.

Why

Owner-directed: a 404 page devs will screenshot. Reuses site tokens (light/dark aware), terminal styling from the landing, and product vocabulary.

How

• components/NotFoundReport.tsx (client): culprit line reads the real window.location.pathname at runtime (404.html is static); all lines occupy space so the reveal never shifts layout; desktop terminal + mobile verdict variants via responsive show/hide.
• app/not-found.tsx: server wrapper; Next emits it as 404.html under static export.
• Cherry-picked onto main after feat(seo): technical foundation - structured data, OG cards, bundle diet, FAQ #20 merged (branch was stacked).

Testing

• Build emits out/404.html containing the report
• Dev verified at bogus paths: live URL pinned as culprit, light/dark checked
• Product untouched: lint pass, 300 tests green

Checklist

• Exactly one logical change in this PR
• make lint passes
• uv run python -m pytest is green (300 passed)
• Docs updated if user-facing
• CHANGELOG entry - n/a (site only)
• Conventional title