feat: Choice Ad placement with 50/50 A/B test

[leo-gravity]

Summary

• Adds a new Choice Ad placement type that renders 4 ads as side-by-side cards (CTA button + domain per card, hover effects, theme-aware) instead of a single banner
• 50/50 A/B test: users are deterministically assigned to either the existing banner or the new choice variant via SHA-256 hash of their userId
• Gracefully handles partial fills — if Gravity returns 1, 2, or 3 ads instead of 4, the cards expand to fill the available width

Files changed

File	Change
web/src/app/api/v1/ads/_post.ts	A/B variant assignment, requests 4 placement IDs for choice variant, stores all ads to DB
cli/src/hooks/use-gravity-ad.ts	New AdData union type, choice ad cache, fires impressions for all choice ads
cli/src/components/choice-ad-banner.tsx	New — 4-column card layout with calculated widths, uniform height, hover effects
cli/src/chat.tsx	Renders ChoiceAdBanner when variant is choice

Before launching: Gravity dashboard setup required

@jamesgravity — Before enabling this in production, 4 new placements need to be created on the Gravity publisher dashboard for Codebuff:

Placement type	Placement ID	Description
below_response	choice-ad-1	Choice Ad slot 1
below_response	choice-ad-2	Choice Ad slot 2
below_response	choice-ad-3	Choice Ad slot 3
below_response	choice-ad-4	Choice Ad slot 4

These should use the same placement type (below_response) as the existing code-assist-ad placement. The backend requests all 4 in a single API call for users in the choice variant. If fewer than 4 ads are available, the UI adapts automatically.

The existing code-assist-ad placement remains unchanged and continues to serve the banner variant for 50% of users.

Test plan

• Create the 4 placement IDs listed above on the Gravity dashboard
• Verify choice variant renders 4 cards when all placements fill
• Verify partial fills (1-3 ads) render correctly with cards expanding
• Verify banner variant is unchanged for users in the control group
• Verify hover effects work on each card (border + text highlight)
• Verify clicking a card opens the correct clickUrl
• Verify impressions fire for all visible choice ads
• Verify credits accumulate correctly for choice ad impressions

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