query-engine: remove the unused sealed-epoch machinery from SimpleMapStore #870

Follow-up to #869 (review comment on global.rs).

Since #869, CircularBuffer no longer seals epochs: every window stays in the current MutableEpoch, whose distinct-window set is exact, and the oldest windows beyond the limit are evicted. No other policy seals either, so the sealed-epoch machinery is dead code in both stores:

• the sealed_epochs: BTreeMap<EpochID, SealedEpoch> field and its read paths in per_key.rs and global.rs (range, exact and batch-exact queries, diagnostics, ReadBased cleanup);
• SealedEpoch, MutableEpoch::seal and SealedEpoch::unique_windows in common.rs, the last two currently #[allow(dead_code)];
• resolve_exact_windows's sealed_epochs argument.

Remove them so each store holds one epoch per aggregation and retention is "the N newest distinct windows" by construction. Behavior shouldn't change, since the sealed maps are always empty, but compare range-query latency before and after: sealed epochs were sorted Vecs, kept for scan speed.