_webui_sidecar_lineage_messages_for_display stitches a session's pre-compression
snapshot parents for full-transcript display. A cleared fork child could resurrect
its ORIGINAL (non-fork) parent's transcript when that parent was later compressed.
Now, when the root session is itself a fork (session_source=='fork'), the stitch
walk allows fork-sourced snapshot parents but stops at the first non-fork
ancestor — so a compressed fork continuation still shows its own pre-compression
turns, while an ordinary fork child stays isolated from the original parent.
Additive guard (one extra break condition); non-fork lineage display unchanged.
Contributor stage; gate-pass. Co-authored-by: rodboev <rodboev@users.noreply.github.com>
