The offset+limit loop walked the IS NULL set, but every batch's writes
shrank that set, so batch N+1 with offset=N*BATCH skipped over the rows
just filled. A 17k library backfilled only 9k before the loop walked
off the (now-shorter) NULL set.
Replace with a tail-recursive pattern: keep selecting LIMIT BATCH on
the NULL set, tracking rows that won't ever resolve in a `stuck` set so
the loop terminates instead of spinning on them.
28738acb56