Freezes the session-list cache source-stamp on the SET of active stream ids (not per-write
state) so mid-stream sidebar polls hold the cache instead of rebuilding all_sessions() every
poll and contending for the global LOCK the streaming worker holds (the ~2 tok/s + 15s
/api/sessions cause). Structural mutations still invalidate via publish_session_list_changed;
only a streaming session's own title/message_count lags <=TTL. routes.py + 109-line test.
99 session-list/sidebar/cache tests pass.
Co-authored-by: nesquena-hermes <agent@nesquena-hermes>
Co-authored-by: nesquena-hermes <nesquena-hermes@users.noreply.github.com>
