When a session's compression lineage spans multiple segments (linked via _lineage_root_id from api/agent_sessions.py), the sidebar previously rendered each segment as a separate top-level row. Adds _collapseSessionLineageForSidebar() that groups by lineage root and keeps only the most recently active tip per group, with a _lineage_collapsed_count marker for future UI affordances.
Co-authored-by: Dennis Soong <dso2ng@gmail.com>
Dennis Soong