Pre-launch hardening: sitemap, robots, build cache, CLS

A focused hardening pass before the next ship window. Five things landed together:

• Sitemap: assertion that URL count is positive at build time, retry on transient failures.
• Robots: explicit allow/deny by environment so staging cannot be accidentally indexed.
• Build cache: split per-app so a marketing change does not invalidate the admin build cache.
• CLS: removed two layout-shift contributors on the home (an avatar placeholder and a font-swap flash).

None of these are individually dramatic, but together they buy us a more predictable production posture.