FIFO session eviction in tui_gateway/event_replay.py deleted the session's
_replay_next_seq, so a session revisited after eviction restarted at seq 1,
under every client watermark, and session.events.since returned nothing new.
Keep the seq counter for the process epoch and raise _replay_evicted_through
to the last stamped seq, so an old watermark gets truncated: true instead of
an empty, complete-looking reply.
The client half (acting on truncated: true) is
https://github.com/NousResearch/hermes-agent/pull/126988.
12c3d5c968