U3: Seed product-managed Events Roundup format

[breadback00-dev]

What is the work

Add the first controlled format: a product-managed Events Roundup format optimized for Bristol/local-media roundups while supporting fewer or more than 10 events.

Planned areas:

• lib/formats/
• convex/formats.ts
• components/formats/
• app/studio/page.tsx
• convex/formats.test.ts
• components/formats/*.test.tsx

Why does it matter

Controlled formats are the product primitive. They give the agent intake lane a target, drive slide generation, and prevent the app from becoming a generic blank-canvas design tool.

What does done mean

• Events Roundup can be selected from the studio entry surface.
• The format defines purpose, item expectations, required/optional details, slide roles, readiness rules, and fit budgets.
• The format works for fewer and more than 10 events.
• Format metadata drives both intake targets and slide population rules.

Acceptance criteria

• Editor can select Events Roundup from the format entry screen.
• Format accepts fewer than 10 events.
• Format accepts more than 10 events while preserving order.
• Project creation is blocked or flagged if required format metadata is missing.
• Tests prove format metadata is consumed by intake and slide-generation rules.

Blockers

• Blocked by U1: Scaffold Next.js, Convex, and test harness #1.
• Blocked by U2: Define core Convex domain model #2.

What can make this fail

• The format is too rigid and only supports exactly 10 events.
• The format is too abstract and becomes a hidden format-builder project.
• Required vs optional fields are unclear, causing intake and editor review to diverge.
• UI presents this like a template gallery before the first format is proven.

How should Codex verify it

• Run format unit tests and component tests.
• Verify the studio entry route can create/select the Events Roundup format.
• Add fixture coverage for <10, 10, and >10 event cases.
• Confirm no user-facing format builder is introduced in this issue.