The recovery fold scanned the generation log from generation 1 at every
open, on the open's foreground — O(whole history) on long-lived brains
(measured at two minutes of a large brain's open). Now an advisory mark
records the log's head whenever the pending set drains to empty (and at
clean close when empty); recovery scans from the mark + 1. The mark is
advisory and monotone-safe: stale-low costs a longer scan, never a
marker. The fold itself moves behind the doors as a latched background
task — the embed worker starts when it settles, and awaitPendingEmbeds()
and close() wait on the latch first, so no caller can observe a
half-recovered set. A pending embed's outcome was always eventual;
moving its recovery off the foreground changes when the worker starts,
never whether a marker is honored.
Pinned in tests/integration/pending-embed-low-water.test.ts: the drain
writes the mark and the next open scans from mark + 1; a pending embed
enqueued after the mark survives an unclean stop; open arms the fold as
a background latch the barrier waits on; a clean close writes the mark
even without a drain.
