tick

Compute and record the next PEX delta against current, which is the peers we should be advertising right now (already filtered by the caller to "proper bittorrent peers we're willing to share", as send_peer does in libtorrent). Flags on each current peer should be built via PexFlags.build.

Returns the BuildResult to encode and send. After this call the builder's internal "known" set reflects exactly the peers in current that fit under the maxAdded cap (plus those already known), so the next tick diffs correctly.