Review follow-up. The viewport-scaled lookahead was unbounded, so a tall window
prefetched several feed pages on every mount / filter change before any scroll —
wasted work on the app's heaviest query. Clamp it to MAX_LOOKAHEAD_ROWS, and move
the policy into a pure readAhead.ts (readAheadLookahead / shouldPrefetchNext) with
unit tests covering the floor, the scaling, the cap boundary, and the fetch trigger
— the core of the feed-scroll fix now has coverage instead of only manual E2E.