Stage-2 D1 contract item (co-frozen): a fact scan may be slow, never
silent. batches() now races its FIRST pull against
SCANFACTS_FIRST_BATCH_MS (10s, exported; test-overridable) — a wedged
or unreadably slow store produces a loud abort naming the contract
instead of a consumer hanging indistinguishably from progress (the
production shape: a heal against a generations-backlogged brain
wedged silently on the first segment read).
Only the first pull is raced: the bound is time-to-first-batch (proof
the producer is alive), not per-batch pacing, and it runs only while
a pull is pending — consumer think-time between pulls never counts
against the producer (pinned).
Three pins: wedged-store loud failure within the bound, healthy scan
untouched end-to-end, slow-consumer immunity.