Add mutual contacts as a compatibility signal

[willdaly]

Summary

The whiteboard identifies mutual contacts as a useful input. The current MVP does not appear to model shared social graph signals.

Scope

• decide how mutual contacts should be represented in profile or match data
• determine whether this is a ranking signal, explanation signal, or both
• surface mutual contact context in results if present

Acceptance criteria

• the data model can represent mutual contacts or shared connections
• matching can account for mutual contact information
• results can explain when a match benefits from shared connections