Keep one snapshot per session instead of one globally

Indexing used to discard every existing snapshot, so concurrent users
wiped out each other's landscapes. Only the same session's previous
snapshot and workspace are replaced now.

main.go

@@ -519,13 +519,15 @@ func (s *Server) indexJob(id, root string) {
 	s.jobsMu.Unlock()
 	snapshot.ID = uniqueID("snapshot")
 
-	// Only the latest snapshot is kept; older workspaces are deleted.
+	// Each session keeps only its latest snapshot; its older workspaces are deleted.
 	s.snapshotsMu.Lock()
 	var oldRoots []string
-	for _, old := range s.snapshots {
-		oldRoots = append(oldRoots, old.Root)
+	for key, old := range s.snapshots {
+		if old.Owner == snapshot.Owner {
+			oldRoots = append(oldRoots, old.Root)
+			delete(s.snapshots, key)
+		}
 	}
-	clear(s.snapshots)
 	s.snapshots[snapshot.ID] = snapshot
 	s.snapshotsMu.Unlock()
 	for _, old := range oldRoots {