A refresh racing an in-flight delete/archive resurrected the just-removed
row: refreshSessions repopulated $sessions straight from the backend page
ignoring tombstones, and the projects.tree prune dropped a tombstone the
moment its id left scoped_session_ids — so the grouped lane un-filtered it
too. The row only vanished on a later refresh once the RPC finally landed.
refreshSessions now filters tombstoned rows (and their lineage tip) before
merging. The prune keeps a tombstone while its mutation is still in flight
locally via $sessionMutationsInFlight, then hands it back to the normal
scoped-based prune once settled.
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