## Summary
Implements the **Traffic axis** of the repeater usefulness score (#672).
Does NOT close#672 — Bridge, Coverage, and Redundancy axes are deferred
to follow-up PRs.
Adds `usefulness_score` (0..1) to repeater/room node API responses
representing what fraction of non-advert traffic passes through this
repeater as a relay hop.
## Why traffic-axis-first
The issue proposes a 4-axis composite (Bridge, Coverage, Traffic,
Redundancy). Bridge/Coverage/Redundancy require betweenness centrality
and neighbor graph infrastructure (#773 Neighbor Graph V2). Traffic axis
can ship independently using existing path-hop data.
## Remaining work for #672
- Bridge axis (betweenness centrality — depends on #773)
- Coverage axis (observer reach comparison)
- Redundancy axis (node-removal simulation — depends on #687)
- Composite score combining all 4 axes
Partial fix for #672.
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Co-authored-by: meshcore-bot <bot@meshcore.local>
