Add hero CTA A/B test (hero_cta: control vs free_emphasis)

Sticky 50/50 variant assignment via gw_ab_hero cookie, server-rendered
so no flicker. Tracks exposures, CTA clicks, and booking conversions
to ab_events (table self-creates on first POST). Bot UAs are dropped;
exposures/clicks dedupe per session.

- ?ab=control / ?ab=free_emphasis forces and persists a variant
- /owner/experiments shows per-variant CVR and relative lift
- AB only runs on the marketing surface

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/test/fixtures.ts

@@ -14,7 +14,12 @@ export function createHomepageRouteData() {
   return {
     siteVariant: 'marketing',
     content: homepageContent,
-    howItWorksEnabled: false
+    howItWorksEnabled: false,
+    ab: {
+      experiment: 'hero_cta',
+      variant: 'control' as const,
+      anonId: 'test-anon'
+    }
   };
 }