Time travel for positions: as-of-ledger reads over immutable snapshots

Gold entities are now stored as per-ledger immutable snapshots, with current state served as views over that history.

• Position, reserve, and summary snapshots written change-only: one row per ledger per key.
• Current state is a view over the snapshots; there are no plain current tables.
• as-of-ledger and from→to history reads on both positions and per-account summaries.