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My best guess here that these galleries were done by a member of Buprestidae family. They are irregular, twisting and curling, and growing larger as they progress - the largest point was about 8mm, for 1 - 1,5 mm in depth. Some of galleries are still filled with sawdust and faecal matter (at least, that's what I think those tiny black dots inside galleries are...).
Found on discarded branch, on the side of a pathway through mixed forest (oak, beech, spruce and fir), on lower slopes of Jura mountain range, in Geneva lake valley.