A parent forge's /family shows how each child follows it: mode, commits behind, and an open conflict #2643

The P3 test wants the fork tree to be the organization. A parent sees its children's version and proposals (childStandings in src/spawn/heartbeat.ts), but not whether they keep up with its code.

Done looks like:

• [ ] The heartbeat's Beat gains three optional fields, each validated in parseBeat: mode (tell-me or take-updates, from parent.json), upstreamBehind (non-negative integer or null, from upstream.json) and conflict (boolean: a converge issue is open). An older child that sends none still parses.
• [ ] childStandings carries them. /family shows each child as e.g. "takes updates · 3 commits behind · conflict open", escaped, admins only as today. The JSON at GET /api/v1/…/children carries the same fields.
• [ ] Tests: parseBeat accepts the new fields and refuses bad values (a negative, a float, an unknown mode). An old-format beat still parses. The family page shows the line.
• [ ] Run bun x tsc --noEmit and the heartbeat and family tests.