Expand description
2026-09-25: The portable file backend: one pinned bounce buffer, a
positional read through crate::tier::pio (pread/pwrite on unix,
seek_read/seek_write on Windows) and a host-to-device copy per group.
HighSpeedSwap uses it on every target but Linux.
Owner: metrale-storage high-speed swap. Invariants:
- The bounce buffer is refilled only after the stream sync that follows its previous copy.