0391 — FanOut's scan bound is a scalar subquery, not a join on old_values

Context. FanOut’s materializing statement scans id > committed from the mark held in the old_values CTE. How that bound reaches the scan changes the plan shape entirely.

Decision. The bound is written as a scalar subquery — id > (SELECT committed FROM old_values) — never as a join against old_values. Joining plans the bound as a join FILTER that walks the whole index from 0; measured 660x slower at 200k rows.

Consequences. The scalar-subquery form lets the planner use the bound as an index start condition, keeping ticks flat regardless of log size. Any edit to this query must preserve the scalar-subquery shape; the join form is a silent performance cliff that only shows on large logs.