CheckDiskSpace (internal/engine/diskspace.go) refuses a download before
writing when its expected size wouldn't leave a configurable reserve free,
so a download never fills the filesystem to 0 mid-write (which corrupts the
partial file). Wired into all three downloaders ahead of any write — torrent
(DataDir), debrid (outputDir, resume-aware), usenet (outputDir, fresh only).
Reserve from downloads.min_free_disk_mb (default 2048 MiB) via SetMinFreeBytes.
The manager treats an InsufficientDiskError as terminal — no source fallback,
since another source would fill the same disk — and surfaces the clear message.
Best-effort: unknown size or a stat failure doesn't block (ENOSPC stays the
backstop). Also hardens formatBytes against an exabyte-scale out-of-bounds panic.
