The _add_unattributed_bytes function was adding the same cross-hour
delta to both days when the interval spanned midnight, violating the
spec requirement to preserve measured volume once as shared boundary
evidence.
Store the unattributed bytes only on the day where the interval starts
(the earlier day). This ensures the total across both days equals the
actual delta without duplication.
Closes#88 (cross-day delta duplication portion)
