Add HAR 1.2 sidecar recording for HTTP exchanges

[ryanfowler]

Summary

• Add --har PATH to record the final HTTP exchange in HAR 1.2 format
• Capture request and decoded response bodies, headers, timing, remote IP, and unary gRPC data
• Reserve and atomically commit HAR destinations with conflict and sensitive-data safeguards
• Document supported modes, limitations, and security considerations

Testing

• Not run (not requested)