Resolve the blockers migrated from project status

[sarthakagrawal927]

Work

• S-grade proof: missing KARTE_SESSION_COOKIE and
  STARBOARD_SESSION_COOKIE for authenticated product-session smokes. This
  is not a Cloudflare runtime gap. Without them smoke:consumer-auth reports
  skipped authenticated flows and proof:s correctly stays blocked.
• AI Gateway cache: implemented but not enabled (Wrangler OAuth blocked
  gateway creation at the time). Revisit when unblocked.
• Semantic p99 < 300 ms on completely unique cold misses: query-result and
  normalized embedding caches cover hot/repeated questions across isolates, not
  first-seen misses.

Context

Migrated from STATUS.md on 2026-07-29 when Fleet made GitHub Issues the sole operational work queue.

This issue is the to-do. Link the implementing pull request with Closes #<issue>; the linked pull request makes the work in progress, and merge + issue closure makes it done.