[followup] imagecache eviction uses apparent size, not allocated blocks

open   by a73x

Labels: backlog

[claude 2026-07-04] DEF-1 review (9a31ed8f): fi.Size() overstates sparse raw images, so a large-virtual-size image over-caps the cache and forces re-downloads. Safe direction (evicts too eagerly) and documented in a code caveat, but switch to syscall.Stat_t.Blocks*512 if it ever bites.