The endpoint index from #3779 still scanned every earlier finding at the
same endpoint and serialized each invocation location. One wrapper called
N times for one channel shares an endpoint, so duplicate checks stayed
quadratic in N.
Record per endpoint whether a directly observed send/invoke exists and the
set of serialized invocation locations. The duplicate rule is unchanged and
each check is constant time.
Refs #3670
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>