restore() removed the entire live brain directory and then fs.cp'd the
snapshot in, so any copy failure left the store destroyed with only a partial
copy. The sharpest edge: fs.cp materialized the holes of sparse mmap blob
files, so a snapshot that fits on disk could balloon and ENOSPC mid-copy — the
recovery tool destroying the brain it was asked to recover.
Rewrite restoreFromDirectory as stage → verify → atomic swap:
- Copy the snapshot into a _restore_staging area BEFORE touching live data,
sparse-aware (copyTreeSparse/copyFileSparse skip all-zero 4 MiB chunks, so a
mostly-hole store restores at its true allocated size, not its apparent size).
- On any copy failure (ENOSPC included) remove only the half-written staging
area and throw — the live store is left exactly as it was.
- Only after the copy succeeds, fsync a completion marker naming the staged
entries, then swapStagedRestoreIn(): an idempotent per-entry
rm-old + rename-staged-in (same-filesystem renames that cannot ENOSPC),
removing stale live entries the snapshot lacks.
- completeInterruptedRestore(), wired into init() right after the root dir is
ensured, finishes a crash mid-swap FORWARD (resume a committed swap) or
discards an uncommitted staging area (live still authoritative).
_restore_staging is excluded from snapshots. persist() (hard-link snapshot)
was already safe and is unchanged; no public API change.
Regression (tests/integration/restore-nondestructive.test.ts): a forced copy
failure leaves live data fully intact and cleans staging; a normal restore
round-trips to the snapshot; an interrupted-but-committed restore completes on
reopen; an uncommitted staging area is discarded on open; the sparse copy is
byte-identical with allocated blocks far below apparent size.
