Function overlays_for_start_at 

pub fn overlays_for_start_at(
    o: Overlays,
    start_at: &OwnedRow,
    sort: &Sort,
    reverse: bool,
) -> Overlays

Drop an add/remove that sorts strictly before start_at (overlaysForStartAt, memory-source.ts:806). Reverse-aware.

Comparator invariant: sort must be the comparator the downstream start gate (generate_with_start) applies — the connection sort — so “overlay dropped here” ⇔ “its committed twin would be gated out anyway”. Narrowing under the constrained index sort instead is wrong whenever start_at does not satisfy the scan’s constraint (a FlippedJoin per-IN-entry sub-fetch forwarding a Take bound): the comparison is then decided by the foreign start row’s constraint columns, and the in-flight edit vanishes from the maintenance fetch — the take-over-flipped-fetch bound loss (follow-ups/take-flip-bound-crossing-edit.md).