Walk an ordered deposit/withdraw event stream against a snapshot-timestamp
stream, returning the cumulative net_deposits at each snapshot. Events
with event_ts_ms <= snapshot_ts count toward that snapshot; events with
equal timestamps are applied in the order they appear in events.
Short helper to mask a wallet address in log output: keeps the first 6 and
last 4 hex characters, so operators can correlate errors without logging
the full identifier.